For Luffy, there is a justification; he can, if he wants, restart his heart and resume fighting. However, he didn't find it necessary to do so against Kizaru but did against Kaido. Perhaps there is a lack of motivation on Luffy's part, as during the fight against Kaido, he mentions the names of Pedro, Tama, Kinemon, Momo, and others who lost or were risking their lives to save Wano. Kaido, in this aspect, surpasses Kizaru because there was much more at stake in the fight against Kaido than against Kizaru, especially since the beginning of the arc focused on showing all the destruction caused by Kaido in the country of Wano and the suffering of the population. This not only motivates Luffy but also all the other crew members to bring down Kaido. Here, he wasn't motivated against Kizaru; initially, he was playing the entire fight and holding back. Kizaru had no reason to hold back and was motivated to fulfill his mission. He even received reprimands from Saturn afterwards for failing. In fact, if Saturn hadn't arrived to save Kizaru, any of the other Straw Hat Pirates could have finished off the immobilized Kizaru after Luffy's punch (Sanji with his Haki). However, the crew's theme is not like that. If it were another Yonko, Kizaru would already be dead by now.
In summary, Kizaru is worse off than Luffy because:
1. Luffy could restart his heart and return to G5 faster than the time Kizaru took to finally recompose himself.
2. Kizaru is a glass cannon who fell with just one blow from Luffy, which, narratively, was only used when Luffy was reaching his time limit. If the blow had been executed earlier, Kizaru would be in serious trouble now, but this doesn't deny that Luffy was indeed superior in the confrontation.
3. Yes, the Luffy vs. Kizaru fight serves to prove that neither Kizaru nor Luffy can defeat Kaido in a 1v1, but Luffy has a better chance of defeating Kaido than Kizaru.