I won't repeat what others already said. I just add that they haven't left forum shortly after getting airdropped merit. Merit system was introduced in 2018 and users in topic that you use as example become inactive way before it. Back then there were no merit system and they were just Hero, Legendary, Sr. Members without merit stuff.
I tend to forget that a lot of newer members here don't realize that the merit system wasn't always a thing, that it's only been around for just over 3.5 years. OP already got his answer, so I won't repeat it. I'm sure if you go back to threads from 2010, you'll see that most of the members who posted have merit counts that represent exactly what was airdropped in Jan. 2018. And man, if you go back that far, it'll seem like the forum was a completely different one. There weren't signature campaigns back then, and I think the moderation was a lot less strict.
3.5 years already? Damn, time flies so fast...