When they show the excerpt from "Stab" with Luke Wilson playing Billy Loomis, Billy is shown slapping himself on the forehead and saying "Stupid!" after arguing with Sidney, played by Tori Spelling in "Stab".
But in the original "Scream", and in the fictitious "Stab" this happens AFTER Sidney walks away. Now, if that is the case, then the only way the fictitious Billy could know that the actual Billy did that would be if the real Sidney saw him doing it by turning back after walking away.
I know this is silly and confusing but WTF, it's not as bad as some questions here, right?
There's plenty that doesn't make sense with the 'Stab' franchise in general. Some scenes from Stab (including the one you mentioned) contain close to the original scenes even though som of those scenes featured already deceased characters and the makers of the movie franchise would not have known every little detail in order to remake it. Unless they asked the deceased people exactly what happened to them or when Billy slapped his head. It doesn't make much sense.
I just rewatched the movies (the tv series is being way too disappointing, I needed a breather) and thought the same thing – Sidney wouldn't know that Billy did that so there's no way she would be able to mention it to Gale so she could put it in her book. But I wouldn't consider that a goof. I actually think it was more of a way to reference the first movie without thinking to much about being realistic or not. But that's just my opinion, I don't believe it was confirmed to be a goof or a reference.
They should have made the series a continuation of the franchise, much like Scream 4 (somewhat detached from the trilogy, but following its footsteps), having the new killer being someone influenced by all previous Ghost Faces, their appearance on the media and even the Stab movies. That would even help getting Neve to appear in the series, if they wanted that to happen. But no, we get a new killer influenced by a lame ass murderer who's killing people we don't care about (Noah's argument really didn't help their case, huh...). But we'll see what they'll do with the plot, they can still save the show. I think.