Thread:AClockworkOrange/@comment-24354745-20131229223124

Note that when Gideon asks Bill "Who are you? How do you know my name?" that Bill doesn't answer the first question. Gideon then calls Bill a demon, so it doesn't seem like Gideon is sure. Journal #3 also seems to have very little information on Bill, and refers to him as "creature" before "demon." Demon is most likely an assumption made by the author, who also isn't certain what he actually is.

TV Tropes also explains this well:

''Eldritch Abomination: Very quirky and affable, but still very alien and incomprehensible. Even his moniker as a "demon" is rather superficial.''

Bill is also very much unlike traditional demonic forces, he much more closely resembles a Lovecraftian abomination of some kind, especially considering the dream/mind-related powers. Wording is important and there's still a lot of ambiguity surrounding Bill. Better to say "supposedly a demon" than claim he IS a demon for sure, when we aren't completely sure. 