A "summary reversal" means the Court didn't even want to hear further argument on the matter; Citizens United is the law of the land, and Montana isn't special. The Court's decision is here, and here's what the one-page per curiam decision states in pertinent part:
The question presented in this case is whether the holding of Citizens United applies to the Montana state law. There can be no serious doubt that it does. See U. S. Const., Art. VI, cl. 2. Montana’s arguments in support of the judgment below either were already rejected in Citizens United, or fail to meaningfully distinguish that case.Justice Breyer, for the four dissenters, says that Citizens United was wrong at the time, and ...
Moreover, even if I were to accept Citizens United, this Court’s legal conclusion should not bar the Montana Supreme Court’s finding, made on the record before it, that independent expenditures by corporations did in fact lead to corruption or the appearance of corruption in Montana.Given the history and political landscape in Montana, that court concluded that the State had a compelling interest in limiting independent expenditures by corporations. Thus, Montana’s experience, like considerable experience elsewhere since the Court’s decision in Citizens United, casts grave doubt on the Court’s supposition that independent expenditures do not corrupt or appear to do so.
Were the matter up to me, I would vote to grant the petition for certiorari in order to reconsider Citizens United or, at least, its application in this case.