Their description of the problem, suggested solution to the problem, and claim that this is the same issue that occurs with other phones contradict themselves. Bumpers do not change where you hold your phone; you are still covering the same area with your hand.
I'm not by any means an engineer in the relevant field but I do understand simple logic. The only reason a bumper could possibly fix the problem with all other things being left unchanged is because this is an issue of where your skin makes contact with the antenna.
I don't understand how anyone can suggest a case will fix the problem while at the same time try and pass it off as if the problem is caused by you covering up the antenna. With a bumper you're adding more layers. If anything it should stay the same or get worse - not better.