So let me get this straight. It doesn't look like Mahoraga's adaptation played a role in this final attack, so what did Sukuna mean by "cutting the whole world"? Is there a proper translation of Sukuna's dialogue?
I can only think that Sukuna's power is like Lille's from Bleach or something? Instead of cutting what's immediatelly in front of him (and thus never surpassing an infinite space) is he able to instantly slash everything in a straight line? "Infinity and beyond" being instantly cut in two all at once instead of there being a projectile?
It sounds like a weird thing to save for the last moment.