Why wouldn't he be a top tier when the 1.5b Admiral level WCI Luffy was oneshotted by base Kaido while Zoro with only a 320M berries bounty did this?
Luffy was a "Fifth Emperor" only according to Big News Morgan. We've already found out that doesn't mean shit. It's the World Government who recognizes and assigns the Emperor title, and three billion looks to be the bare minimum bounty for the WG to give that out. Nice try though.