Oda made Garp lose his stamina by fighting the Blackbeard Pirates' commanders instead of giving Aokiji a solid victory in a one-on-one battle. The feat of capturing Garp is something that was achieved collectively and not individually by Kuzan. Whatever the difference in power between Kuzan and the nearly 90-year-old elder Garp, Oda is telling us that it wouldn't be easy for Kuzan, and the Blackbeard Pirates' interference and assistance only served to expedite the process, who knows how long it would have taken for Kuzan to defeat Garp without the assistance of the BBP, or better yet, we could be surprised with Garp winning as well. That is still uncertain.