This was really odd for me to learn only a couple weeks ago. I had always assumed I was a Gen Yer and that Millenials were just getting out of college now.
You're correct. None of the parent's argument makes any sense in light of this error. I don't understand why the post is being upvoted--the data is outright wrong!
I sense that people may blindly upvote posts reporting figures, especially if they come with sources.
Generation X and Millennial isn't really a sharp transition. No generation is, but it is even more diffuse this time since there wasn't necessarily one or multiple "big events" at a fixed date. I don't know if parent's argument is correct, but what is being said is essentially early Millennial, ~1985 and late Millennial, ~1995. Huge difference in many e.g. housing markets. Once people born around the actual millennial gets old enough we might start talking about Generation Z instead.
Could be the case for many people. I think the commonality between generations might even be declining in favor of other divides. It is in some ways assumed that many people are starting at the bottom and mostly share paths based on development. But I am not sure that is the case anymore, if it ever really were.