Hot Take: A character being able to fight Special Grades does not make them Special Grade.
This shouldn't be considered a hot take in the first place, I just diskike the way the community treats the grade system.
It's not to be taken at face value or neccessarily indicative of overall strength. Kusakabe is said to be physically weaker than Nanami and Mei Mei and lacks overall power compared to them, but he's still considered a stronger Grade 1 due to his versatility with no CT.
Several of the cast fight Special Grades but are not labeled as such, this leads to people incorrectly making the assertion that the they are then Special Grade. When in reality, a sorcerer of any grade should be able to punch above their weight class, atleast in the case of curses.
I mostly see this with Shinjuku Yuji, ppl often regard him as being a special grade by this point, when he'd be more closer to a Grade 1 sorcerer given the circumstances of him fighting Sukuna. Just becauae he has a domain and RCT does not automatically make him a Special Grade. And just because he "fought" Sukuna doesn't make him one either.
This also applies to Hakari(who's implied to be as strong as a Special Grade even though he isn't one) Ryu, Uro and Maki. These characters either aren't ranked at all or aren't ranked Special Grade, but their abilities and what they've been shown to do suggest they could handle special Grades. The community takes it upon themselves to label them as such when Maki is literally still a Grade 4 Sorcerer.
Ultimately I'm not going to ignore that the Grade system takes strength into consideration, but in the same way there are variations in how strong a Special Grade is, the same can be said for the other Grades, which is what makes the system so nuanced in the first place when you don't reduce it to a power level term.
(This didn't need to be so long I just like Yapping)