You both are probably right I forgot this was the android app. Meta has a habit of sending everything they could possibly ever get incase someone wants to buy it. Although I could see them using that for BI purposes to be like X Users were wearing headphones on these types of posts, but that may be giving them too much credit.
Also if you click one of the users who haven’t signed up yet it just pre-queues your follow and says “You’ll automatically follow then when/if they join threads”(I don’t remember the exact wording here). This is blown out of proportion.