601.2e does give a chance for a spell, once announced, to be undone. I’m not sure how relevant it is, but I’d think Seth could weave it in a way to argue the spell, as proposed, was illegal in some manner by making an illegal choice with it intentionally.

601.2e The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 730, “Handling Illegal Actions”).

Personally I don’t care a whole lot about the outcome though. Seems like neither player was bothered, and if they’re happy, I have no issue with it.