The point is he was named an Emperor after WCI for his achievement+ Grand fleet,etc. And then you Had BB telling Luffy he isnt ready for the title yet. So what exactly is different with the Buggy case?
Becoming Yonko/Emperor wasn't necessarily about Strength Level, rather power/influence/political ties.
Thats probably why Buggy and Shanks moved up, they probably had artificial feats and/or information traded for their position
Luffy just beat 2 commanders and had islands + a fleet, they didnt even know if he could sky split
Thats why Emperor title has been diminishing in value ever since
This is just my take. Im just trolling the fake shanks boys that would gatekeep mihawk from being a top tier