Here's what probably happened. The manager saw that her staff member was in the weeds and had a hard time keeping up. She took your drink and appetizer order, to help her server catch up. Unfortunately sometimes, restaurants are understaffed for unforseen reasons, and I think it's awesome that the manager would step in and assume doing table service duties. (I wish mine would!!!)
If she did not say, "This will be on us.." or something to that effect..do not assume.

Most restaurants want you to be happy with the food and service...we do what we can. We want you to enjoy yourself and have reason to come back. We are not mind readers, however...if you are not happy..say something! Usually we have no problem accomodating you if you have a complaint or concern. It does not sound like you voiced your displeasure with management, though. It sounds as if you made an assumption, didn't get what you assumed was coming to you, and then wrote a complaint letter afterwards.

Had you said something to the manager, I'm sure they would have done something. Especially since it was a corporate restaurant.