Think of it this way: you believe in the Christian God not because you've seen him going around smiting people with a giant hammer and giving you free cookies, but because you have faith that he is the overarching figure of our universe that requires worship simply because he is what he is (rather than 'because he created the universe' or 'because he knows all', specifically). There are arguments that if God came down and started smacking people left and right, there would no longer be any question of faith or religion; you would simply relate to him like you do to an oncoming tsunami, or Donald Trump, or your dad. In such religions, the most important aspect of a divinity is not their ability to swing hammers the size of mountains or to send people to heaven and hell; those capabilities are simply adjuncts to the fundamental fact that there is nothing higher than a god, there is nothing before a god, god defines the basic boundaries of the world that cannot be violated, surpassed, undercut.
When you learn that those gods are (1) created a few hundred years ago, and therefore they are not even as old as the Engwithans whose ruins we live in shadow of, (2) they were created by mortals like ourselves, (3) they are not fully sentient beings with powers of intellect, wisdom, cognition that easily outstrip mortals, but more akin to animals or machines in that they are preprogrammed with certain biases, tendencies, instincts, responses, and run with them in a very sophisticated way, then it is very easy to see that (a) no, it is nowhere as earthshattering as learning the gods don't exist, and no, it is unlikely to make all mortals in the realms immediately abandon worship and go secular; (b) yes, it is going to have seismic changes in the role of religion and how people relate to the gods all over the place.
Why would that be the case, if the gods are still existing and powerful beings? (i) The fact that the gods are relatively recent creations undermines the special authority that being a god gives you; if the Hebrew Bible dictated that Yahweh did not create the world, but instead was created by wise proto-Ur-hebrew humans hundreds of years before Moses, that produces a very different attitude to worship. (ii) The fact that the gods were created by a people whose machines still litter the land gives rise to the very real possibility that gods can be newly created, modified, destroyed, as suits the mortals - a possibility which was only being raised as a "holy shit, could it really be?" speculation by mortals in the wake of St. Waidwen and the Godhammer. Before that, the only known case of a God being killed is the whatever-his-name-was Hephaestus knockoff in an altercation amongst the gods themselves, in the style of Greek myth. There is a massive difference between a world where people worship the gods as all-powerful beings who have always been there, and a world where powerful gods roam but can be created, modified, killed, by mortals. You can very well expect that, in a timeline where the POE protagonist reveals this secret to the world, you will see a different history over the next few centuries as massive splits occur between people who continue to worship the existing gods and wish to try and retain the integrity of the original pantheon, and those who seek to discover the power to create new gods, some of them perhaps successful, all of which becomes compounded by the fact that (iii) the gods are preprogrammed rather than omniscient and wise beyond human ken, meaning mortals now realise once they understand how a given god works and what makes them tick, you can easily manipulate them, even set them against each other, i.e. enrol them in your machinations - and in fact, Durance becomes an unwitting test case of this principle, where he has accidentally found a way to make himself invisible to Magran. All in all, what you find is a situation where a society that treated its pantheon as the limits of their world, things which could not be disobeyed, tricked, modified or defied, goes through a massive revelation - Waidwen saga + Watcher's expose - and then becomes a society split between those who seek to maintain that tradition, and those who now believe that the gods are things that mortals can control, defy, fight, create, etc.
It's actually a typical modern atheist's one-track thinking to just say "so what if the god is fake he's still super powerful and shit so nothing changes at all", because obviously, for the atheist who is unwilling to believe in divinity to begin with, there really seems to be no distinction between an incredibly powerful superhuman being and any kind of god, and the 'proven empirical fact' of seeing him swing his hammer around obviously supercedes any superstitious 'belief'. The comment that POE gods being created is 'more logical' than the perpetuity of the Christian god, for example, is utterly irrelevant to the question of whether the POE revelation makes a difference to the people in that setting and their existing mode of religious relation to their gods.
(P.S. A more relevant question is, we have cases like Greek mythology where the Olympian pantheon is basically a third generation pantheon, and they've got a lot of other pseudo-divine beings that they're birthed from or have fought and imprisoned, so what gives? We know that the Greek attitude to their own gods, from the beginning, was very different from the Christian and other monotheistic religions. The Greeks didn't rebel against their gods and seek to restore Titan rule or whatever. The answer here is that the Greek religion was from the beginning formulated in this way, crafting a set of dispositions that allowed for humans to sometimes partially escape from or cut deals with the judgments of the divine, and depicted the gods not as omniscient limits of the world but more like the very top of the food pyramid within our world. The thing is, it's not logical to say, "look at the Greeks so of course nobody gives a shit if their gods turn out to be made or whatever". The question is, "would Christians shrug and carry on if their God turned out one day to be more like Zeus?" Or "would the Greeks have shrugged and carried on if the Olympian Pantheon turned out to be constructs created by Mycenaeans a couple centuries before the Classical Age?" The point is that in the POE world a stable religion with a very clear set of relationships and attitudes developed, and then you have a Big Event that overturns large portions of that theology while leaving large portions intact also. That's the interesting part, because now you're going to see schisms and splits between people who draw new conclusions about the gods based on new info, and those who try to conserve old ones, and people inbetween, and heck people who opt out and go atheist/agnostic/whatever.)
