No one is saying you did anything wrong Ste, and I am sure you were careful...but I think the point the fella in with the disco might have been trying to make is that if the ground is soft then a mud tyre will do damage by its very nature. Granted there were ruts already there, but if the ground is soft a mud tyre sinks in, and then the tread blocks drag the car along...they need to sink in to work, thats one of the reasons that muds are not so good on the road. Noone would part their truck on muds on their lawn at the moment because of the mess they would make for that very reason.
People are passionate about their byways, and the number of them that get closed every year, and there are some idiots who relish getting as muddy as poss, driving repeatedly through the same sticky bit of byway until its wrecked.