As it happened, the plane was announced as further delayed to 6:40, then made better time than that, and eventually left at 5:55. So I was right in trusting the first delay notification, but not the subsequent one.

The delay was caused by the lateness of the plane that was on a previous route. So once it arrived and they did a quick clean, we were on our way. Luckily, we didn't have crew timing out the way other flights did.