Actually, that's wrong. You can arrive at MCO via yak and still use DME. As long as you are listed on a WDW resort reservation, you get to ride DME! I've arrived at MCO in a rental car, coming from Tampa, I've been dropped off at MCO. I've always been able to get on the DME bus. Your DME reservation isn't tied to your MCO arrival..it is tied to your WDW resort reservation.
So....if you are listed on a resort reservation, that is active on your MCO arrival day, you just have to have made a DME reservation, prior to arrival..then, you're good to go.