G4 timelimit ain't that big of an issue against BM, with gear stacking, complete immunity to lightning, and FS, there isn't anything logically stopping Luffy from only fighting in G2/3 for as long as he needs to bring BM's hp down, and then finishing with the final G4 assault if needed. he will be giving her the beatdown of the century, and she does not have the AP or Speed to even compete. Luffy was straight up just tanking Drunk haki amped Ragnorak level attacks and coming right back at Kaidou.
There is not a single attack BM has that competes with the above, nor is there a single CQC BM attack which she can land like that against Luffy. BM doesn't have the speed or the AP to compete, while Luffy has everything in his toolkit to be breaking Big Mom inside out.
BM cannot operate at this level in CQC.
This is what BM looked like after some brief fighting and a couple attacks from awakened Kidd and Law, despite having the ability to heal.
BM vs Pre G5 Luffy entirely depends on if Luffy is fighting properly, or if Luffy wants to be a complete jackass. BM is at the mercy of Luffy at the end of the day, even ignoring Kaidou's statement confirming Luffy's superiority, he's just a bad matchup for BM, he has internal attacks amped with ACoC, so her best asset in durability is easily bypassed, while he is immune to lightning, can casually tank much stronger attacks than she can dish out consistently, and has FS to outmaneuver her ontop of already being faster. There's really no reason for Luffy to fight in G4 let alone G5. G4 can be activated when she's on the brink of her HP and stamina, after taking hundreds of ACoC attacks.
I don't even want to think about what Luffy will do a weakened and drained BM with that final G4 assault. He will turn her into mincemeat.
Everything checks out here, Kaidou wasn't just talking out of his ass.