"Propelled" is the other term in that definition. A conveyance might be mechanical in the sense you describe without being propelled by a machine.

At the same time, an individual person being blown forward by a sufficiently large fan might meet the qualification of "mechanically propelled" without being in a mechanical conveyance per se.

But more generally, a vehicle plus a motor of some description would seem to meet the definition. ICE, steam, electric, spring-wound, whatevs.