I think the user ID is being cast to an unsigned integer, 0, therefore assuming we're dealing with a guest user.
It's really quite an unexpected thing to happen; usually the user bb code would be generated via a user mention where there's no chance of this happening, but clearly it could be confusing.
Might be worthwhile to have a better error message... If you go to a user profile with a user ID under 1, you get the same message about it being posted by a guest. What exactly was posted by a guest here? lol