A different site covering this same story mentions that the methodology used actually involves the computer scanning something like 18 million youtube videos, s

A different site covering this same story mentions that the methodology used actually involves the computer scanning something like 18 million youtube videos, simulating what areas in the brain each of these videos would be likely to affect, and comparing that to what was recorded during the subjects' fMRI sessions. So the watermark-like words are coming from the youtube clips, not from the subjects' brains themselves.