Lessgo full buddhist mode:
Sukuna explained it to Kashimo: no matter how physically strong/perfect was Gojo, he had a psychological weakness, namely his (human) emotions. Gojo really cared for his students and actually loved them. Same for his team mates. He also (somehow) cared for the sake of humanity. He felt remorses toward his friend and mate Geto Suguru. He felt anger toward the jujutsu high elders. He took care of kid Megumi, then Yuji.
In other words, he was "cursed" by his own feelings, or more precisely by his sympathy/empathy toward other ppl, because care/love inevitably implies and brings fear, remorses and anger.
This is a fundamental point here, cause in buddhism those emotions are the root of and what cause samsara: the cycle of birth, death, and rebirth that all sentient beings are believed to be trapped in. It is characterized by impermanence, suffering (dukkha), and the absence of a permanent, unchanging self (anatta).
By feeling those antagonistic emotions (love/fear), Gojo stayed trapped in samsara, preventing him to reach true enlightment by reaching the nirvana, a state of liberation, freedom from suffering, and the end of the cycle of birth, death, which implies the extinguishing of the fires of greed, hatred, and delusion.
On the other hand, Sukuna managed to get rid of any feelings toward (almost) anything by (almost) transcending his human condition.
As he explained it to Kashimo this chapter (and before to Yuji and Jogo), what made him the King of curses was his (partial) understanging of what love is, or most precisely, that love/care (for other) is a worthless and restrictive illusion that only weakens ppl by keeping them trapped in samasara.
By (partially) freeing himself from it, Sukuna (almost) reached the nirvana, hence the true enlightment. The status of an (half) god/buddha.
Partially. That's fundamental. Cause that's the core point of my peak fiction theory.
Let's go back to the actual reason that made Gojo lose. Was it his love/care for the others? Was it fear?
Nope. If Gojo actually loved and cared for his students and the sake of humanity, it's his selfishness and greed for acknowledgment and reciprocal love that caused his defeat. (As you can foresee it, Gojo's not the only one to be selfish here...).
Gojo's ego, self-confidence and cockiness made him let his guard down for the result we all know. I insist: it is not his emotions for the others, but his massive ego.
In buddhism, getting rid of empathy, sympathy, love for the others (what Sukuna managed to do), isn't enough to reach nirvana and get truely enlightned.
One must before all supress his own ego/ egoistical self and become "selfless". Selflessness is the only path toward true enlightment and absolute empathy/pure love (emotionless love).
And there we come to Sukuna's weakness: does he have suppressed his own ego/egoistical self?
Nope. He's the embodiment of absolute egoistical self/egoism/greed.
This is why he's not THE enlightned, but the fallen angel. He almost reached the nirvana, but was rejected because of his absolute egoism and self-love.
Now how can it be a weakness toward Yuji?
Simple: he will nerf himself for the pure sake of being entertained by him, the one he liked to humiliate and bully for his own fun.
For some (almost obvious) reasons, he likes to bully and humiliate Yuji. He could have killed him many times, but he didn't, because Yuji's his favorite toy, his fav distraction. Because by the end, no matter what he can say, Sukuna feels alone, and he needs the others for his entertainment, for his greed, for his egoistical self.
Accordingly, he will restrict/nerf himself to enjoy his fight against his fav toy, letting him get stronger in the process. Because once again he lives for that: his own fun/entertainment.
And this is his weakness. His ego/self confidence, as for Gojo, IS his weak point.
He will eventually let his guard down, allowing Yuji to cast his soul-related CT and somehow defeat him.
Why all the buddhist shit then?
Because Yuji is the true enlightned one. The only honored one by the heavens. Not Gojo, not Sukuna, but Yuji.
Why? Because he's been selfless since his birth. Yuji embodies the buddhist absolute selflessness, therefore the absolute altruistic love and empathy (compassion).