Does mhanbostanci or the other accused came with explanation or anything, except this info they left in the negative feedback?
The signed message is pretty strong evidence, you can't go around.
In the Turkish thread from 2015 mhanbostanci claimed that one of those accounts belongs to a relative, they shared a computer, and accidentally posted from the relative's account. Unfortunately in some other thread mhanbostanci also pretended to not know the alleged relative and even attempted to do a lending deal with said relative. A classic busted sockpuppet story.
https://bitcointalk.org/index.php?topic=1247502https://bitcointalk.org/index.php?topic=1206095