Nowhere does Barone say that 30% is a decisive win. He finds it significant only because Romney was expected to do much worse there.
You are reading things into Barone's article that he did not say.
Romney wins in either Alabama or Mississippi—or, in what everyone a few days thought was impossible, both—could effectively eliminate Gingrich from voters’ consideration and could make Rick Santorum’s path forward seem so unforbidding that even this determined and indefatigable candidate could find it hard to go on.
In lieu of quoting all that, I was simply using the word "decisive" which is synonymous with what he wrote and a fundamentally laughable analysis from someone who should know better, probably does, but has RINO masters to appease.