And why on earth would he admit it later??

Effective journalism. Someone asked him the right question in the right way, and he wound up volunteering something he might not have otherwise chosen to say.

That said, I think some of this is just reasonable openness and accountability. Given the game was effectively a full year ago and the issues with VAR from that game are still a somewhat spicy topic, it's obviously something he's been questioned on rather a lot - and given that PGMOL also released a statement, and he mentions that he spoke to on-field ref after the game ... it seems like he's already owned that mistake professionally and personally, so making the same acknowledgement of error publicly isn't that huge a step beyond that.

It was definitely a mistake that the fans deserved an accounting of, considering it's the exact sort of problem that VAR exists to address - and that it should have been a call, of some sort at least, was absolutely undebatable to anyone who'd seen it.