If Metal Knight (with prep) scaling to 6-C with Flashy Flash and Platinum S is a no-go because of Flashy not reporting things, what about High 7-A?
High 7-A would basically set him above any lone hero with the exceptions of Flashy Flash, who's capabilities are unknown to the masses, King, Tatsumaki, and Blast. The latter three aren't really imagined as people anyone could match up to either, so I think it makes sense that they're not considered for that statement.
It'd come from Homeless Emperor and Golden S, who the association definitely got reports about. We know for a fact that Zombieman told the association about Homeless Emperor thanks to chapter 173. Stitch is even the one who asked Zombieman about him first. And plenty of people saw Golden S.