I flagged this answer as "Not an answer" because it was just a "thank you" answer at that time, but has been edited before the moderator review (to a worse copy of the referenced post). The flag got declined. Should I flag for moderator attention in these situations?
Yes, you should flag "thank you" posts. This was an edge case where the edited post looked like something that also frequently gets flagged, a short or duplicate answer. When the moderator reviewed the flag, it looked like a legitimate (although low-quality) answer. The non-answer "thank you" post really should not have been edited this way. The revised answer doesn't add anything to the page.