Details matter and the article may have them wrong, but my reading is this accidentally covers out of state cars as well. I'm not going to look up the law to see who is right though.
The cars are registered in MT but they’re under CA jurisdiction if they are primarily driven here. That’s the literal loophole. CA is not trying to force people to pay CA registration and sales tax on cars kept and used primarily out of state.
That is the intent of course. However it is really easy to write a law that missing important details and so despite the intent cover all cars regardless of where they are used.