They might have been intercepted by military. It is doubtful that they would have been shot down, but it would have been legal to do so.
The big difference is: No one forced them to land at a specific location -- they could have turned around and fly all the way back to Russia had they desired to do so. They landed in Austria because they decided to go to a neutral country and that they wanted to avoid NATO. It is fairly obvious that not letting someone enter is very different from letting enter and then coerce.