So what stops Hybrid Kaido from engulfing his fist with his magma-like fire in order to put holes into people just like Akainu?
Hybrid Kaido could engulf himself in this magma-like fire too, in order to punch holes into people like Akainu did. Like, he can generate an island worth of these flames to surround himself in it. And that is just a side power of Kaido, just like lifting islands is a side power. So the powers of 2 admirals (Akainu, Fujitora) as side powers, when his main power are his insane physicals, including his durability, endurance and recovery. Plus being able to create storms, tornadoes (which Zoro referred to as a natural disaster), scythes that cut through everything, lightning roars, amped by top tier Haki and his advanced club moves, which hit immensely hard (knocking Luffy out multiple times, even killing him).