He never doing so is one, but okay I take you word he would try to summon KH for this match.
Well, Demigra have his Aura that is a passively affecting Sora body.
1- His Aura have layered EE - 7 layers (Which Sora would just ressurect from It, but If he does so, he just gonna get erased again and again)
2- Demigra have passive stamina reduction, so he can just steal Sora stamina and make him unable to do anything.
3- Passive layered corruption (ALL types), Mind, Deconstruction, Corrosion Inducement, morality and Madness hax.
Since they all passives, Sora gonna get hit with all this effects which would making fighting Impossible for Sora.
Well, Demigra also have Imessurable LS vs Sora Class M, so he can just hold him down with LS or his chains, so Sora can't move.
He can also just seal Sora in the Time Labyrinth, a realm which is outside of time (sounds familiar?) and whoever is there, gets suspend in a time stop.