Yes, but that statement only qualifies him for Strongest in that 300 year span, not necessarily what comes after.
I asked initial, and like
@Dalesean027 said, it's basically case by case, but I asked because we see the Statements in the movie and WoG but I want to know it's going to be used to try and cover Supes retroactively.
Together, yes, they mean Superman is the strongest metahuman (Active or Not) in those 300+ years, but logically, that doesn't cover anyone who becomes a Metahuman after the events of the movie. Of course, "within reason", Clark would scale over Street Level Metas for example, but Jane Doe randomly gets a Tier 7 or 6 feat and no context exists to scale Supes over it, WoG shouldn't retroactively scale him as it's made in reference to every Meta in that 300 year span.