The WG does think that Buggy himself is a top tier though
They had the idiot imprisoned in Impel Down after he was hauled in by Smoker lmao.
They know very well how powerful he is.
The World of One Piece's perspective =/= the WG's perspective since they know more than the World of OP.
Besides, the Yonko titles are given by the common folk, not the WG.