This term is a weird thing.
Not only Metroidvania was supposed to only tell how the (back then) modern Castlevania game design was quite like Metroid, Castlevania was already going that route almost right in the beginning with Vampire Killer and Castlevania 2.
Metroid wasn't a franchise yet so people probably didn't even think those games reminded of Metroid. Hell, Metroid had just been out for a little while before Vampire Killer so Metroid probably didn't influence it at all.
So basically Castlevania went back to almost its roots with Symphony of the Night and games like that and added more "rpg elements" to it and suddenly it's "like a Metroid game."
Maybe this was addressed in the video, I haven't watched it yet. Or maybe this was already addressed in this thread, haven't read it yet, lol
Oh well, I have accepted that term long ago and it's a good term to tell what the playstyle is.