[Tactics]

Originally Posted by Ka-el

Spoiler:

I doubt Fuga is about RCT especially because how fire is the opposite of cutting? Until now we saw that RCT applied to a CT is the mirror opposite of the CT, like Blue is pulling and Red is pushing, like RCT of Anti-Gravity is Gravity. It would make more sense if RCT of cutting was mending, for example.
Also he didn't say that a cursed spirit wouldn't understand but that a cursed spirit wouldn't know, in fact Sukuna said that he though it was known that he had flames, so probably he expected it to be recorded in some books of history and so a cursed spirit of course wouldn't know about it, especially if it's not a very ancient cursed spirit.

Spoiler for :

Look at pattern (scissors and line) made by Yuji before he cut the pillar.
Also sudden narrative that "CT appearances changes based on era due to difference of understanding."

Its similar pattern as "Cut" function in Paint software, which opposite is "copy and paste" object that already touched by it.
Sukuna can use flame not because he knew Fuga is flame but because he knew the outcome of Fuga will be Fire, result of copy-paste Jogo ability. Opposite of to cut is to mix, unite or blend, after all.