How so? ORAS was developed at the same time as XY. If they wanted to put in the ORAS Megas into XY, they could have, but decided to leave them out.
Likewise, they could have out all the ORAS megas into the demo, but they could also have decided to leave them out.
There's no way of really telling which way they went with it, and a quick browse through this thread and the one that came before shows just how successful people here are at predicting what Game Freak will do. Of course, all the Megas could have been revealed by now and the ones in the demo could be all the ones we're getting. But we can't use the Demo as a definite because we have no idea if Game Freak does have more Megas and simply took them out for the Demo. We've had a lengthy discussion of it and it could really go either way.
I highly doubt there'll be any new megas, unless it's built on an old build or they're still making new megas. If they have megas that aren't used in the demos, then they probably have all of them