In no way is that physics. Sure, a bouncy ball will bounce less in mud than it does on concrete, but in both cases a bouncy ball (your Cain) will still bounce higher than something soft like a pillow (your Baxter). The pillow does not have an advantage in the mud.
Furthermore, foot strikes do not work like bouncy balls. It is all about pushing off the ground. If you push harder than the other person, then you will be faster. Mud or no mud. Mud affects everyone in the same way. THAT is physics.