Only thing I can imagine buggy being able to do is convince some seriously strong pirates to join him that we never seen and somehow got his hands on a ancient weapons. Buggy gag moments were funny even him becoming a warlord was ok but him somehow faking his way to being considered a yonko is going to have to be some major story tell for oda cuz it just doesn't make sense.
By comparison BB had to beat ace the WB and take his df. To finally officially obtain the title he had to take most of wb's territory and take out the WB pirates. For Luffy to become a yonko he had to take out a warlord, 2 yc and beat a yonko in a clash, then finally he helped take out 2 yonko and was a core part of it. What could buggy have really done to match this? And nothing could have been that big because characters have always been updated to world news. Yet the characters know nothing of buggy's big accolades I hope oda does start making plot holes.