He used AdCOA on Kaido, it was too shallow according to Luffy himself. I agree with the rest, though I don't think "Luffy held back" theme applies to Kaido when we literally saw him use G4 and still fail to do anything to him. So the only logical explanation is either Luffy's base has gotten a boost or Kaido is just weakened or this is like one of those dumb humiliations that BM had to do through like others pointed out.
No he didnt mention internal stuff when he said too shallow, it was vague etc so i don't believe that. Especially when his Red Roc bruised Kaidou (a sign of external damage instead of internal). Even with G4, the impact was external. Kaidou was pushed around by external force, but no sign or art depiction of internal damage, but the external pressure was much more dominant (Kaidou pushed away from Luffy, but Luffy damaged the tree and the impact penetrate through the tree without damaging the tree's front side), etc. Luffy stalled Kaidou+BM with unending stamina while the other rooftop fighters struggled to just stand, AdvCoC Ragnarok literally inflicted minor damage to Luffy so its a sign of Luffy's actual durability, Yamato said Luffy's so strong and he still held back...i could go on but tldr i'd just say Luffy saved his energy in order to make sure he can beat Kaidou when he had the surefire-kill weapon, CoC Coat skysplitter.
...the other alternative for me would be plot restriction or Oda kept Luffy's powerlevel down to the trashcan, so he can jump it back up when it matters the most lol