So you need to be very careful in being happy about this:
All the court said was that the dealers have no private cause of action. That is, they can't sue to block tesla themselves.
I'm not so sure that's true. The conclusion is that the statute "was intended and understood only to prohibit manufacturer-owned dealerships when, unlike Tesla, the manufacturer already had an affiliated dealer or dealers in Massachusetts." I think there would have to be a third-party Tesla franchisee in Massachusetts before the AG could sue on their behalf.
That reasoning will be held to be not dicta on the standing issue of private dealers, but would probably be relitigated if the AG sued :)
This is because the legal conclusion (under the heading "Conclusion") is limited to the dealers and their private right of action, and thus, only the reasoning that applied to that is considered stare decisis.
BUt otherwise, you may end up right. Courts tend to be finicky about this stuff, depending on circumstances.
The state AG/etc could still do it.