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.