Lets isolate this all down to just one motion: you have just stepped with your right foot and your left foot is on its way over. Actually, lets make this even easier and make it walking. So your right leg is straight out and your left is taking its time coming forward. Slow your image way way down. When you are walking on normal ground, you engage your right gluteus maximus (and related muscles) much more than when you are walking on a treadmill. Can you see it? While you are on the treadmill, the function of your right leg is much more “balance your body while the treadmill takes your foot back to simulate walking motion” than “propel your body mass forward.”
Yes, your left leg is doing the same amount of work in both cases, and YES, your are still moving away from an arbitrary point on the treadmill, and I think this is where most people get hung up on the “its all relative” train of thought. As for the Earth=megatreadmill argument, that doesn’t need to be mutually exclusive with my explanation above. Take the treadmill from my example. Now slow it down to half its speed, half of that, until it is moving at whatever rate the earth is spinning at. Thats it.
edited to clarify.