Yeah people definitely go overboard with that. I think the only real problem is that Arch-based distros might be using other repositories and not be completely sync'd with Arch itself so then users will go to Arch support forums or communities with problems that don't affect vanilla arch. But I've never really cared when people using derivatives claim they're on an Arch system.

Installing isn't even as big of an undertaking these days too, so it's less of an achievement then people would like to think. Last time I did a clean install on my laptop I just ran the 'arch-install' script included on the vanilla ISO and it was super easy, lets you choose where you want it installed, pick from a list of desktop environments, and you pick between alternatives for other common packages then you're good to go without having to do much manual work during the install itself.