I'm gonna say this, because I would imagine this is why it is worded that way:
Out of the 2 main features of DME upon arrival, the first is the ride from MCO to the WDW resort.
The other is the luggage transfer. And that is going to be the sticky point for someone who doesn't take DME in the "conventional" way of fly into MCO, use the yellow tags, get a ride to the resort, then get your luggage.
If TAs start telling people they can fly to Sanford, transfer to MCO by cab or other means, there is bound to be confusion about whether or not they should use the yellow tags and have their luggage delivered. (WE frequent visitors and experienced visitors know better....but you see how much misinformation and jumping to conclusions happens on here, much less what we don't hear about.)
There's enough confusion as it is...so I'm sure they are trying to keep it simple to say, "can't use DME"....rather than..."well, you can... but.....you have to get your bags, and you have to get to MCO, and you may or may not need to make up a flight arrival time, and ...how would you get to your return flight?"
Sounds like way more trouble than it's worth.