Guess by that logic magnets shouldn't work? Because there is nothing to push on.
There already exists examples where you don't need an actual physical material being expelled in order to have thrust. The key in this example is they are saying they have reached a force great enough to counter gravity.
There already exists examples where you don't need an actual physical material being expelled in order to have thrust. The key in this example is they are saying they have reached a force great enough to counter gravity.