I don't know how exactly people define what is or is not a thing for the purposes of "pseudo-elements applying to things" , but my first expectation was that, by design, :closed can not be the same as :not(open). The former would in theory apply to things, and only to things, that can have a state of closed-ness and are in that state of closed-ness, whereas the latter should be able to apply to things that may not have the possibility of being open or closed.
But if that's not the case, then I don't really see a point for :closed existing, yeah.