I've always thought "generational" was very much a personal description. Everyone has different ideas about what a "generation" in hockey even is, much less who belongs. For me, it's what players come to mind when I think of a 10ish year period. I think I would categorize it as being similar to "the face of the NHL" for a period of 5 or more consecutive years.
It's not just being the best player for a long time, I think of it more as someone who's style and personality on the ice shape the entire game while they're playing. When someone thinks hockey at that time, they think of that player. Arbitrary start and end dates, though, and there is a LOT of time where, in my mind, a generational simply wasn't there.
Richard for the 40s, Howe for the 50s-60s, Orr in the 70s, Gretzky in the 80s, Lemieux in the 90s, (BIG maybe for) Jagr in the early 2000s, Crosby and Ovi in the late 2000s, early 2010s, and McDavid from 2016 to now.
Matthews is a solid no right now. I'd argue this is his first year of "generational-level" play, so he's got a while to go.