I think, to me, what separates the RPG from most genres is narrative-driven consequence and choices impacting the story, world, progression, and/or character. The thing about genres is they tend to lend defining characteristics from each other. What separates an adventure game from an RPG, for example? Adventure games often have similar mechanics and gameplay methodologies applied to them. Both can be focused on story, as well as contain character progression mechanics that are pretty similar. I think most adventure games tend to stray from providing options that impact the story or character, though. AC and RDR2 are pretty good examples of that. Sure, RDR2 you can make choices that impact the overall end game arc and how the world responds to you in pretty artificial ways, but nothing that makes a real impact outside of the end story. There are aspects of these present, though, such as one of the last missions in chapter 5. But they aren't much of a focus.
Things like this aren't as clear as some other genres, but even then you see people mixing up the meanings or definitions. An MMO is a great example. People calling Destiny or Fallout 76 an MMO really seems to miss the "massive" aspect of the genre, but it's still becoming more and more acceptable to lump those games together because they share some core mechanic or design similarities.
Ultimately genre definitions are something I've always had a bit of a hard time with, because different people see different things that speak to them. It's especially prevalent in music genres, such as punk rock, that have so many sub-genres that people frequently argue over. It's way more abstract in music, though. A lot of people use it as a way to gatekeep which I think we can see in the example OP experienced where people narrowly define an RPG to be a very specific set of games that came out in a very specific generation. As technology gets better and the limits of games gets pushed further, I think the lines between some genres will become more and more blurred.