It is perfectly obvious to me and to the vast majority of people. That is why they didn't bother defining it further.

> An ox & cart fits the bill for "machine" with that lens

No it doesn't. I don't think you thought this through. The cart itself cannot do anything without something else acting on it. An ox is obviously not mechanical (it being an animal) which is what is propelling the cart. Therefore it is not mechanically propelled.

If it was a person a bicycle then would be more ambiguity. But it is commonly understood that a bicycle (excluding e-bikes which are mopeds) is not a "motor vehicle", because it is propelled by the rider.