... it's only function in the setting is being mentioned.
This is practically the function of 99% of all PnP setting material. The fact that the Harpers and other fuckers are actively curtailing technological development is
in itself evidence that the setting doesn't at all "ignore it completely". Just because you don't have widely available firearms across the entire planet doesn't mean it's not there. Even in the real world, our oldest sources on black powder are from the 100's, and it took another 800 years before it started seeing real, practical use in warfare in the 900's, and even then it didn't really take off until it intersected with north-European metallurgy in the 1300's, and
even then it took another good 200 years until we're starting to see widespread use.
Look, I like the Arcanum aesthetic, I would absolutely love to see some magic-infused gunslinging, I want to have top-hats and rune-inscribed bolt-action rifles, and I hope to one day mow down a legion of medieval communists with a gatling gun that shoots magic missiles, especially if they look as cool as they did in The Temple of Elemental Evil.
But to say that there cannot be legitimate reasons why things would develop differently in a completely different world - let alone one with magic and alchemy to obviate or curtail development - is just stupid. Especially (pre-4e) Forgotten Realms is a silly example, because there are literally hundreds of wizards in any given metropolis, all of which will ask you to piss off with your toys, and a host of deities that flourish under the status quo.