it was not if he had to fight 5 days and then used Gas to defeat them. even if he didn't wanted to kill them he could defeat them. defeat doesn't mean killing. and on Onigashima the moment he was defeated and Kaido has to protect him was shown and he was in his Zoan form.
Onigashima is dimly explained. If full Zoan is his tank mode, he may Switched into it when they started their strongest attack, for pure defense.