Chechnia was not a reconized independent country like the Donbass now. That is why Russian soldiers who by accident wander into Ukraine don't fight - and the Russian soldiers on their own side of the border probably will not shoot either if they meet wandering Ukrainian soldiers. But Russian soldiers actually fighting are in a dubious position. But the suggestion is that those are a minority among the Russians fighting in the Donbass. Most of the Russians fighting were said to be former military men. There was some mnention of Chechen men who earlier fought against the Russian army in the Caucasus an now went to the Donbass to help the Russians there.
A connected matter is the original planning of this rebellion. Militaries play all kinds of war games among them regular army against guerrillas. The players can be very emotional about the game - I remember reading in a book about war games ( the Pelican book I bought about fifty years ago ? ) that two US army colonels playing a game against each other in the Pentagon after a few weeks were hardly able to speak to each other. In the US army the purpose of nearly every game will be how to defeat the guerrilla because since the Indian wars that has been the purpose of that army except for world wars and the wars against much weaker countries. Of course they will have gamed the mujahedin in Afghanistan against the Russians but that will have been an exception. The Russians have a memory of the role of the partizans during the Great Patriotic War and so be more ready to be sympathetic to the guerillas. I imagine that the effective tactics used by the Donbass fighters will have been the result of a tradition of dozens of years of gaming in the Russian military schools. And of course of a good knowledge of the qualities of the Ukrainian army or the lack of those.
so I give the Russian denial: 
