Well, actually, his reaction speed is specifically referring to
this. Sukuna is able to chant and fire WCS as an electromagnetic wave is shot towards him and intercept it.
While it's a viable win-con to hit Mori with it, the problem, like I said before, is Sukuna prefers CQC, and he is a very curious person, seeing someone like Mori would stir him up to engage him. This is ESPECIALLY the case because Sukuna, when seeing Maki, who possesses no CE, was spurred on and engaged her in hand-to-hand combat, completely ignoring his RCT to fight her.
There's a good chance Sukuna would be enthralled by Mori's raw physical power and engage in pure hands, if this ends up happening, Sukuna can genuinely get ripped in two.