I'm sure this must be a load of rubbish

I don't think it is meant to be understood the way you think. I imagine that what they really mean is if you back up against an immovable object such as a wall, you may bend the towbar or worse buckle the underbody structure to which the towbar is attached. That would be worse than damaging the bumper if you back up against the wall without the towbar attached. The same goes, by the way, if someone runs into the back of your car.

The way that it was written suggests that it is a poor translation from a foreign language.

I take your point Lutz, but there was a further sentence that I didn't copy onto my post. It states "The dinghy can start to jackknife, forcing the tow bar to bend". I suspect it came from a boating forum.  I think they should stick to boats.