I think this would be true if we had wheels instead of legs, but I'm not so sure the forces involved in our gait are as simple.
A car simply needs to overcome rolling resistance, which would be more or less the same on a treadmill or road (with similar surface).
But people are essentially pushing off, jumping, and landing. On a treadmill, the machine helps with part of the push-off motion since the treadmill moves your landed leg back for you.
I’m going to have to disagree with you here. This is a frame of reference problem.
Work can be measured by the relative motion between you and the ground.
It makes no difference if you are moving over the ground or the ground is moving underneath you feet.
To show you that are true, let me suggest a “thought experiment”. Suppose you slowed down the earth’s rotation to 7 mph. would it be easier 7 mph to run west (against earth’s rotation) or east (with earth’s rotation)? And how is running west any different than running on a treadmill with a belt 12,500 miles wide (with lots of stuff on it)?
Now going back to what you said about “the treadmill moving your landed leg back to you”. On a treadmill relative to the belt, when your foot hits the belt you need to expend energy and push back with your legs or you will fall on your face. This is because your body is not moving relative to the belt. A similar example of this would be jumping out of a moving car. Be prepared to expend a lot of energy when you feet hit the ground just to stay upright.