If you were doing it wrong there might be a problems, but YOU would know if you did it wrong (something about that bouncing that you can't mistake :)) )
A good sitting trot is likely as comfortable for the horse as it is the rider. We post for ourself as much as for the horse. If were not bouncing, then the horse isn't being bounced on, so there's not a problem.
I wonder if the person in question stands in her stirrups when she canters? Granted, I haven't ridden Dressage in oh..... well, over 40 years (and we'll leave it at that), but one of the many things the wonderful equestrian lady who taught me to ride (and of the few that I still remember), if the ride is uncomfortable for you it might also be uncomfortable for the horse to (she was teaching us trotting at the time and she also taught us to do a sitting trot).
I can't imagine you were doing any more damage than riding in general would do. Have never seen anything that indicates that a sitting trot done correctly on proper fitting saddle is any worse then walking or posting to trot.