Ahh, that’s just because you don’t hang out on forums much.
A “thread necro” (and any number of variants) as when you comment on a post that has been inactive for a very long period, bumping it back up to the top of the list – “raising it from the dead”, etc. See here for more info.
So, in this context, he revisited a plot thread from 5-6 years ago.
My only real stretch here was from “forum thread/post” to “plot thread”…not particularly innovative, but still enough to amuse me.