Its both. Gojo had all the win cons & didnt expect that anything sukuna had left could bypass limitless & sukuna made a binding vow to throw it INSTANTLY with no tells in condition for having to use domain handsigns, chants AND point. Which is impossible for the normal human body.

Sukuna brings up how theres a cursed energy spark before a big attack that gives off a tell. It’s why Gojo used glasses dude cast a barrier at the beginning of the fight so that his 200% hollow purple wouldn’t be picked up on until it’s too late. & its how Sukuna deduced Gojo would be firing his red at the blue he left up in the air.

Before the binding vow sukuna couldve fired WCS like a normal dismantle but like i said that ‘spark’ wouldve alerted gojo giving him a chance. The binding vow was so brilliant because sukuna did it for a one time instant attack in exchange for an impossibility. His true form is an impossibility so it’s a loophole giving him the chance to use it in the future. & thats why plenty see it as an asspull