I saw that post.
I had come to a couple conclusions some time ago about that.
A spammer will register here and save a link to a thread, an old thread. Then later even year later they come back thru that old linked thread and post their spam. Another thing is when posting on an old thread there is a chance the spam is missed and soon enough the old thread gets buried again and the spam is there to stay unless by chance we stumble across it.
Just a couple theories I came up with when I was studding the habits of these spammers couple years ago