Putting the matter aside of people linking the "only few people capable of fighting me" with the heads of the guys shown behind Luffy, which from the flow of the panels is not even a given, Kaido is talking about Luffy´s ceiling, not the ceiling of the "top".
He is practically asking how far Luffy can develop. His train of thought is "just a few days ago i destroyed him, what happened in that short while, only a few people are capable of fighting me, how much can he develop further", which implies now Luffy is one of those that can fight him, while asking, since he managed to reach that from not being able to fight him at all to actually doing that, how far he can develop further, and as practical representatives of how far Luffy could develop, you have 5 distinct characters who each made a different mark on the world.
The ceiling is relevant for all of the 5 guys, only on different levels, both in strength, and also achievement.
Is Luffy "just" going to peak at Oden? Is he gonna match a Yonkou, meaning Shanks? Can he reach Rocks, my old captain (who might be an inbetweener between Yonkou and PK level)? Or is he gonna reach PK level strength but not become PK (Whitebeard)? Or is his ceiling actually Pirate King (Roger)?
While we know, his ceiling in strength is at least PK, but in achievement it is actually higher.