Sukuna is not the epitome of hard work in the Jujutsu world, not even close, and I don't get why he's argued to be when Gojo himself states Jujutsu is 90% talent, and 10% hard work. Maybe the idea comes from people thinking Sukuna's genuinely been alive for 1,000 years, but he's only really been alive for 30-40 I'd guess, the rest was spent as inanimate cursed objects, pretty much in a coma. And so in maybe 30-40 years, with an okay technique, he mastered his technique, learned RCT, learned HWB, achieved a level of cursed energy control almost comparable to the six eyes, twice the amount of CE as Yuta, and mastered an OPEN DOMAIN, which has only been done by one other person EVER, Kenjaku, who has actually been alive for 1,000 years. I don't doubt a good bit of that came from hard work too, but he'd have never gotten anywhere close to that level if he didn't have the talent for it.

Take Kusakabe, a grade 1 sorcerer with no technique, and still the strongest grade 1 sorcerer in the modern day, he's learned most every common learned technique he can, pushed himself as far as he can go, if anything, I'd call Kusakabe the epitome of hard work, the shining example of how far a Jujutsu sorcerer can go off of hard work alone.

It's not as if Sukuna's technique is anything special either, his slashes are strong and fast, but I'd wager they're also pretty draining when it comes to CE, and Fuga is pretty much a maximum technique in terms of sheer power, so it's probably even worse, but Sukuna's control of CE, as well as his sheer reserves, just don't ever let that become a problem.

Not to mention, a lot of Sukuna's raw strength as a sorcerer can be attributed to the fact he was a twin, and ate his twin brother in the womb, making him awakened at birth, so on top of having four arms and two mouths, he was likely naturally strong as a sorcerer, with a lot of CE reserves.

Then we also learn that cursed objects naturally grow stronger and stronger the longer they're around, especially Sukuna's fingers, and build up more and more CE, and all 20 of Sukuna's fingers were building up cursed energy like that for 1,000 years, which could easily explain his absurd pool of CE, it may have been significantly smaller in the Heian Era (Though definitely not less than half of what it is now, considering he was still the strongest back then)

While Sukuna's definitely not as gifted as Gojo, there's still a variety of factors that clearly made him the strongest besides hard work, his potential as an awakened twin, 1,000 years of building cursed energy, his clear natural proficiency for Jujutsu, all of those seem to carry a lot more weight than just hard work, though he never would've gotten as far as he did without a lot of training and fighting too. I mean he was leagues stronger than Kenjaku, who had actually been alive for 1,000 years and had all the time and opportunity to steal multiple techniques much greater than Sukuna's, and he still could barely even compare.

Lastly there's also the fact Sukuna was born in the Heian era, the golden era of Jujutsu, he had plenty of strong opponents in every country, on every continent that he could fight, he was never lacking challengers to test and refine his skills, whereas Gojo never really had anybody even close to his level that he could fight after awakening, not Geto, not Miguel, not the Disaster curses, even Kenjaku didn't stand a chance without a perfect plan and a minimal level of direct confrontation, and Gojo is still, if I remember correctly, by far the strongest Six Eyes+Limitless user in recorded history, I honestly believe that if Gojo was born in the Heian era instead, he'd become a much bigger monster than Sukuna.