I know it wasn't me who gave you the merits, but what you described above could be a reason why any old comment receives merits way after it was originally written--just look at all the merits satoshi's posts have earned.  He hasn't posted in over a decade.

Sometimes if I'm doing a post history review for someone or if I'm looking to give out merits to help someone close to their next rank, I'll do a deeper dive than I normally would and sometimes I'll hand out merits for posts made months ago.  It might look unusual if you're paying attention to which posts of yours got merits, but to me it's just kinda normal. 

Plus there could be members reading old threads for educational/entertainment purposes, and they might merit old posts.  I've done that as well.