uhh why exactly can he not be??? whitebeard went from splitting the sky on his medication with shanks to never doing it again? Luffy went from clashing with kaido but not using adv coc right and could not sky split to sky splitting with kaido when he returned? Check the panels man.
Shanks example (He attacks and the fodder are knocked out around him):
Luffy example (Not attacking but flowing adv coc to which people are being knocked out):
Zoro example (Just like luffy not attacking, but flowing adv coc out of swords this knocks fodder out):
Garp example 1 (garp comes flying in flexing adv coc,and instead of fodder being knocked out they react to him shocked):
Garp attack aftermath (they are sent flying nobody knocked out):
Garp attack aftermath 2 ( fodder up talking about revenge on garp again nobody is knocked out garp admits to being weaker):
Garp Example 2 (fodder reacting to the fight and again even with blue hole no fodder are knocked out in the process):
Everything points to him using adv coc,but the fact that his adv coc is not knocking out fodders while in all the other examples they are is definitely saying something. Garp while indeed strong is not as strong as he used to be even using adv coc and being impressive af.