I asked Erictheblue to clarify from this thread before the forum was wiped.
I pretty much explained it in the other thread, but I'll paraphrase to make it clearer.
Citizens United was decided on the basis of the US Constitution.
States have their own constitutions and laws. Federal preemption sets a floor, not a ceiling, on what states can do. (So if federal law says "X, Y, and Z," states have do to X, Y, and Z, but can also do A, so long as A does not contract X, Y, and Z.) It's on this basis that states can outlaw capital punishment, even though the USSC has held that the DP does not violate the 8th Amendment.
Same situation here. If the Montana state constitution has a narrower definition of person than the US Constitution, then by interpreting the state constitution, Montana could define persons as excluding corporations. The holding is only valid in Montana.