My headcanon for what happened is that immediately after someone says "Gojo won", Sukuna began his technique chanting, starting with "Dragon Scales". Gojo knew Sukuna was preparing an attack he hadn't seen before, realised it was a powerful last-ditch effort, and decided to go on guard. Gojo thought that since only Mahoraga could allow Sukuna to negate Gojo's inviolability, and Mahoraga was now dead, all he had to do to block the attack was activate the infinity, so he did so... and THAT'S when he got cut by the World Cutting Slash
