Unfortunately, yes, all cutoffs are based on the scheduled departure time. In addition, a delayed departure may be changed at any time and may leave early than the posted (delayed) time.

I can understand why the policy is in place in many cases -- for example, if the airline is able to come up with replacement equipment or another crew, they don't want to have to wait any longer than necessary.

That said, there's nothing that would prevent the airline from accepting bags after the cutoff if there was still enough time to get them on the plane (as was certainly the case in this instance). I don't know why they wouldn't put your bags on your plane. Their ground staff would have still been there for the 7:13 PM departure to Dulles anyways. (If yours had been the last departure, I could have believed that some of their ground staff might have gone home for the day so they couldn't load any more bags.)

I assume you were still able to fly on Monday. Did your bags make it on the Monday flight anyways, or did they go on Tuesday?