They could be hired from outside the forum. I've seen "bumping services" spam threads, in this case it's shitposting on the tech boards.
What's really disappointing is that they don't get banned!
I think you are right, regarding my story that I quoted below is not only a fictitious story, but a real story and now it is proven to exist in this post
https://bitcointalk.org/index.php?topic=5573822.msg66384835#msg66384835In common sense, if it is not because of the paid bumping service as you mean, why do they buy an account that has been marked with RedTrust.
I think this has something to do with what you mean.
-snip-
I don't know what the motive is, I once saw in a Facebook group that someone was looking for a Bitcointalk account to buy, even though the account marked Redtrust he was also ready to buy. and I believe those accounts have been sold to the buyer of that account.