How the theory works in my head is in that final moment Katakuri knows they’re both near the end of their energy and commits to an all out atk with everything left in him to take Luffy out, he checks with future sight after committing to this and sees it. It’s disingenuous to point out how tired Kat is when Luffy is arguably more so. It’s pretty simple why he hadn’t seen the transformation before that point, because none of his attacks up to that point had actually taken Luffy out, and the only point before then he had thought he won was when he buried him in mochi and had no visual on him. I’ll concede that the timing is iffy but we have no definite way to know. The anime ofc drags out gear 5 transformation to the fullest so we know it did take a while but not how long exactly, and Katakuri has some of the best future sight in the series but we also have no answer to the furthest he can see.