People as usual not reading the context of the entire chapter and the flow, but get hung up on individual statements and treat it as disconnected from everything else.
1) Kaido is angry about Luffy still talking about PK -> he thinks it is audacious that Luffy is talking about it
2) Then he questions how Luffy could have hurt him, and wonders how he could have gotten so far in a few days after he destroyed Luffy
3) There are only a few people who can fight me
4) Now considering Luffy to be one of them, as it is implied considering he damaged him, he now asks where his ceiling could be, where he could peak, where he can develop to. You can consider this as a possible challenge for himself even, considering he had relations with all 5 of them at one point in his life.
Asking about the ceiling, meaning not being able to gauge it, yet at the same time using 5 guys on upposed similar levels does not work, because it would imply he considers the ceiling of Luffy at that level, which would make the question meaningless.
5) To put that to the test, meaning how far he could develop, he does the same attack he destroyed him with to see how he reacts to it, Luffy this time dodges for the most part-> he praises him for that, once again a sign Luffy developed from chapter 923.
6) Kaido once again is active in going after Luffy right after BM, which is a stark contrast to himself in chapter 923 who was drunk due to boredom and let himself get pummeled until he had enough.
7) Kaido literally tells BM to step back because he wants to test them to see where they stand, entire chapter is a test to see if they are worthy to be fought seriously, once again alluding to "where can i place you, where is your ceiling".
8) Killer and Zoro attack, he praises their effort.
9) Captains attack, he sees them as monsters in their own right (opposed to the monsters Kaidou and Big Mom, and also the monsters he faced before, meaning the people in front of him are worthy, no coincidence he called Oden a "monster Samurai"
10) Now seeing them as worthy, his decision goes from "break them and make them my subordinate" to "i will kill all of you" since he now accepts them as challenge, which is finally once again underlined by him pointing out that the winners will be closer to PK, meaning he sees them as rivals for PK now.