I don't think he's stronger than Prime Whitebeard or PK Roger.
It should be other way around tbh.
WB massive hype comes from being equated to Roger.
And, Roger even though was PK but we know it wasn't just strength but having ability to reach to final island and find one piece. Despite that, we haven't seen Roger with massive feats. His fight with WB ended in a tie.
Xebec - this guy has unparalleled feats. Him controlling the crew full of monsters like WB, Kaido, BM, Shiki in itself is the testament of his strength.
Its WB and Roger who needs to show they were as strong as Xebec....not the other way around