Tbh i don't even know how cutting fire is supposed to be a power up for zoro
I mean fire cutting will only be useful against fire users and then nobody else.
I always thought of zoro cutting fire as an increase in skill which will be useful in certain situations and not as a powerup.We know kinemon can cut fire yet he is weaker than zoro,so the only powerup zoro needs to be a top tier swordsman is an increase in coo and higher coa to achieve a black blade.
He may also need an increase in his movement speed to be able to cut anybody in an instant(shunshin style) just as the GREAT RYUUMA
It seems there's a discrepancy between the terms "power up" and "new techniques" and "training". Here's how I see them defined in practice.
Zoro always creates "NEW TECHNIQUES". In Alabasta, he created Shishi sonson to cut Daz. But he also received a "POWER UP" by unlocking the breath of all things to cut steel. Then in Skypiea he was "TRAINING" his breath of all things "POWER UP" to use it better and he did by using it with a "NEW TECHNIQUE" called 108 pound canon to Cut Steel with the breath of all things but now using an air slash.
A "POWER UP" is an immediate shift in base stats like Zoro moves from random fodder swords to Sandai kitetsu and Yubashiri which increase his durability and AP due to their quality. Same with Breath of all things, Zoro moved from not being able to cut steel to cutting steel immediately so that's "POWER UP". Stuff like unlocking a new set of power like unlocking haki is a "POWER UP". Even a new mode is a "POWER UP" like 9 sword Asura mode
"TRAINING" is actually mastering those base stats to improve them. Zoro trains to get used to Yubashiri being light and Sandai Kitetsu cutting shit he doesn't want to cut. Zoro is now "TRAINING" to tame Enma which will improve on the haki he already has.
Notice Enma and Shusui are the same grade of sword so this is no longer an immediate increase in Base stats so it's NOT a "POWER UP"
"NEW TECHNIQUES" are new Configurations of existing powers and stats. Zoro is already able to cut Steel since Alabasta and create gusts of air since Arlong Park so he reconfigures the two powers to cut Steel with air slashes to create a "NEW TECHNIQUE" called 108 pound canon.
Therefore The ability to cut fire, if it's not something based on reconfiguring Zoro’s existing base stats like a "NEW TECHNIQUE" and is not an extension of an existing power through "TRAINING" means that all that's left is that cutting fire is a "POWER UP"
@Jo_Ndule in case you were still confused