Why did a nine-day-old answer pop up in the new user first answer review queue?

Question

here - https://english.stackexchange.com/review/first-posts/269683

(lets do the time warp again)

Answer 1

Because the Stack Exchange garbage-collection daemon — who’s affectionately nicknamed the Roomba — has just had its way with this closed question.

That delete not just the question but an answer from the again-new user, since it brought the owner of the post in your review down below some new-user threshold, which kicked one of their remaining posts into the FP queue.