I didn't read the
study originally, but I saw people referencing it when talking about NPCs. Going though the archives, you'll find that it's based on some article from, I'm not joking, 2011.
Not Everyone Conducts Inner Speech
The tl;dr of it is that a study was done on 30 students in a university and they found that only a quarter of them experienced inner speech and seeing. Why it suddenly became popular, I have no idea. Maybe someone from /x/ (I got the feeling the true origin of this "meme" is from there) dug it up again and spread it to /pol/, as an alternative for "shill" and so.