The Six Eyes doesn’t give you heightened perception speed, it just gives you more to see. Satoru had brain damage, was exhausted and thought Sukuna had no means of bypassing Limitless (Domain Envelopment would be useless with only one hand to fight in a body as feeble as Megumi’s, Makora was exorcised and everything else wouldn’t work)— at the start of the fight, when Gojo had none of these to consider, he BARELY reacted to a regular Dismantle, and even then, it was when Sukuna prepped it with a hand sign.
It makes total sense that even a healthy Satoru would get hit by it, as he’d only maybe have JUST enough time to BARELY avoid death by it (meaning he’d still get hit, just not a fatal blow), which is what Gege said. The fact it was a brain damaged, exhausted, low output, damaged Gojo gives so much more context to why he would’ve lost.
Narratively it makes sense he lost, but
Realistically he would have won, and it makes zero sense as to how he died the way he did