I noticed that some members who aren't actually posting for the first time are showing up...
That's intentional: every user who posts shows up once, because every user can be a spammer.
That means he hasn't posted since I created this topic, and indeed, he just
woke up from October 2020.
It's not the first post from this user (far from it) but in the list you would not be able to tell in any way until you actually go and view the post.
Correct. I'm just surprised how many old users are still waking up to make their first post in years.