I suspect since they mention database corruption that these photos were on your device but not referenced in the database.
Apple added code to recover these photos (including possibly non-deleted ones that you've lost) and didn't check if they'd been deleted.
They played it safe, better to recover something thought deleted than to not recover something that was lost. I agree with this thinking.
At least that's my speculation. Orphaned data recovery that was too ambitious.
For the record I didn't get any resurfaced photos myself.
So the db spontaneously gets corrupted then magically heals itself and a bug allows once deleted photos to miraculously reappear. It’s a bunch of Apple doublespeak.