The issue isn't whether or not you have "already done your thinking" and reached a decision on some matter. Reread the article and note that the experiment consisted of showing the self-described partisans videos of their politicos making contradictory statements.
Statements on topics which I am sure most thoughtful people, and most partisans have covered the biggies, have already done their thinking.
In essence, who cares what the yammering monkey on the screen says, I'm dealing with my own thoughts on the matter.