Maybe they weren’t willing to count his garage as being part of his “occupied dwelling”. There is usually a real key locked, door to the outside style, door between your house and your attached garage.
People in CO shoot folks in their house (including drunks who are just in the wrong house) all the time without being subject to criminal or civil proceedings of any kind, but the garage has to make it at least a gray area.
AFAIK, that usually applies to detached garages.
The MT lawmakers were rushing to repeal the “expanded castle doctrine” upon hearing of this case. So AFAICS, Kaarma was railroaded and may even be a victim of ex post facto.
There is case law, if not statutes, in every jurisdiction that determines what is included in the Castle Doctrine. I don't have my reference handy to look it for Montana right now.
But that's not enough. The Castle Doctrine is just like Stand Your Ground. It just means you don't have to run from your house. It doesn't grant authority to use force. To use force the other elements of self-defense have to be met, or there has to be a proactive law that authorizes lethal force to protect property.