Because it was a genuine thought. She genuinely did intend to remind Caitlin the game started, for completely innocuous reasons. That thought slipped out because it was an honest thought with no malicious intent. It's no different from saying "alright guys I'm going out for dinner, see you all later" in her mind. Scum say things like that all the time, it's just a part of normal posting/interaction. The slip was that she didn't realize the thought wouldn't materialize if she was town, and even if it did, she would react to me calling it out far less weirdly.
For example I think that town Abigail who actually didn't know it was a rule would just say "oh my bad I didn't know it was a rule" instead of saying my post is stupid and not a big deal. It's a very defensive reaction.