Indeed they are, but please abandon the defense of the sentence you wrote, as it is indefensible.

D-u-u-uhh!Some jerk-offs even study campaign lies and think their gospel, too! means, at best, that they study campaign lies and also contemplate their own gospel. ("Thinking about Jesus as I study them campaign lies...").

Your intent was to tell us that such jerk-offs, having studied campaign lies, believe that such lies are as credible as gospel.

Thus, they think or believe the lies are true or that they are gospel. Hence, "they're".

By the way, it was also a very rude sort of sentence, even insulting in its petty way.

A