Maybe the information is being lost in translation, but I'm trying to understand where the "chlorine gas" is being talked about. The articles I found just say "too much chlorine", which is hazardous but not the same as chlorine gas. Can anyone shed some light here?

I don't think "chlorine gas" but rather chloramine. In pools when chlorine binds to contaminates, it forms chloramine, which is a gas that sits on the surface of the pool.