Buggy being part of Roger's crew literally means nothing. Buggy is a fraud who specifically gets by without being powerful, that's his whole gag, he's a weak clown who magically ends up winning in the most ridiculous circumstances, just because that's how Buggy thinks he'll become PK doesn't mean that's how 99% of people will do it. You need to collect the Poneglyths which are held by extremely strong characters and you need to fight others who are also seeking to become Pirate King, that's why Chinjao said you have to stand at the top, only the strongest of those vying for it can pull it off.
Not the strongest swordsman in history, Roger is stronger and didn't have a black blade, and the databook also hints at something other than haki being required, again, reason enough to conclude that it's not this huge power feat.
You're reading comprehensive skills suck.
Buggy was explaining to Mihawk and Crocodile how to become Pirate King. He told them they don't have to worry about building up their wealth and power first. By him becoming PK will automatically gain wealth, fame, and power.
You choosing to ignore manga a facts doesn't make you right.
The whole point to Buggy character is to show us it's sure you don't have to be the strongest to become PK. Nor does becoming the strongest make you PK.
Roger didn't become Pirate King by beating everybody else. He begged WB for Oden. Without him owning he wouldn't ever became PK.
You don't have to collect the poneglyths to become PK, you just need to read them. Guess what, just because you're strong doesn't mean you can read them.
Mihawk has a world's strongest title. Roger doesn't. It is never been hinted or applied he was stronger than Mihawk. The only reason people put them up there is because he's PK that is not a strength-based title.