He's not saying you have no chance per se, just that it would be VERY difficult because of the fact they would have to be built from the ground up.
The way it works for KH, is that you rip the model from said game, then fix o suit FF7's needs. If you were to make Advent Children models, which btw just for reference, imo, would look out of place in FF7's badly stretched universe, you would have to build them from scratch.
Meaning modelling, texturing and such. Not impossible by any means, but very, very difficult.
Still, it would be an interesting thing to see if it were to be done I suppose.