The 3500 pound limit is based on many variables, including braking, safely accelerating on to a highway, being able to handle highway speeds with out the trailer sending you out of control, etc.
It is not based on pulling something out of a ditch at 2 MPH. Sure, if you tie a rope around your bumper and try to pull a 10K pound load out of a ditch, you may rip the hole bumper off your truck. But doing reasonable things should not result in a catastrophic failure of your truck.
FYI, a week ago I watched a Wrangler try to pull a 26K Rental box truck out of a ditch. It was fun to watch the 4 tires on the Wrangler dig 4 holes in the ground, but we weren't going anywhere. However, the F450 that drove up as we were working on extricating the truck had no problems pulling the rental truck out of the ditch.
My Canyon was a few hundred miles away, but without at least half a ton in the bed, it would have been useless in this rescue.