It makes sense once you think about it. So Deku could've been born with DECAY and Inko went to All For One to take Deku's quirk away so Deku didn't cause harm once he was born. Shigaraki didn't manifest his quirk till he was 5 which is when of a kid hasn't manifested their quirk they're deemed quirkless. So after All For One's secret love child, aka Deku, was born, he took the quirk that was already manifested and gave it to Shigaraki. Therefore the lore makes sense