1. Vs Kaido was the equivalent to getting a near fatal bullet wound or car accident. G5 acted as a defibrillator to restart his heart. Vs Kizaru was the equivalent to running a marathon. He’s totally out of stamina but there’s no lethal damage. Really his health is in no danger at all, he just needs time to recover. A defibrillator wouldn’t do anything here.
2. Since he was literally resurrected from the dead against Kaido, Oda could take artistic license to make that work however the hell he wanted, hopefully for the first and last time, and he decided to resurrect Luffy back into fighting shape.