I'm surprised you are surprised. The GA's are pressured (and graded) on getting the plane closed on time. (To an extent so are the pilots). The policy even predates the merger. There is signage in DFW saying they can and will close the door early. If the lobby is empty they can and are going to load standbys and close the door. Quite possibly to probably they
had filled up the plane with whomever was there. Maybe if you have a George Clooney card they hold it for you, but only if there is no chance of the crew timing out.
The plane I was on yesterday was fully loaded at T-20. Except two mobility-impared passengers in the front who did not plan their airport arrival near early enough and were "having trouble" getting through security. Whatever that means. The GA's and onboard crew were fuming and upset to be waiting for them when there was still another flight later. (Except it was also packed).
We still pushed 10 minutes early. I have no idea if they got on or not.