What counts as "extra stuff" is subjective. And what the developer thinks when people say "Don't add extra stuff" could be anything.
This attitude presupposes that developers are sloppy and impressionable, and that they don't really know what they want to make. Which to be fair
is often the case, particularly with high-budget studios who are generally there to sell units, not fulfill a vision.
But to a developer that actually has an idea what their game is going to be, "Don't add extra stuff" is a very objective mandate.
In my opinion, every game should be developed with a strong vision of what the gameplay loop is: how the player is going to be spending their time, and why that is fun or interesting. If there are multiple activities (and there usually should be - even the best core gameplay can get repetitive if not dynamically paced), those activities should be thought out and contrasted against one another, to create an intended experience with highs and lows; and in an RPG, choices about how to approach things. Even a straight up combat only RPG gives you a million choices how to build your party, and different builds should have distinctive gameplay; if they don't, or if a single "optimal" build is required to succeed, the game has failed.
Usually RPGs have a straightforward triangle of activities: exploration (tension building), combat (tension release), and going back to town ("cooling off"). In a well-designed game, each of these is thought out and done with intention to maintain an entertaining cadence. If you throw in a racing minigame as a weird 4th activity, that cadence is thrown off ... again, unless it's done with intention when building the foundation.
When you have an idea to build a 3-legged stool, you should build a 3-legged stool - it's pretty clear what people would mean by saying "don't add more legs".
If you didn't know what you wanted to build in the first place, and you're just letting people suggest 100 different legs to put on your chair, you're already a weak designer and imo you shouldn't be doing it at all.