Theyre rigged in the sense that you're going to get a far right and ultra far right candidate, which is good enough for me to call US elections rigged.
The leaders of the half that believes it know it's true because they did the rigging.
The leaders of the half that doesn't believe it also know it's true, but don't want to admit it because the first half's preemptive DARVO-ing has poisoned the rhetorical well against them. Also, admitting it would just weaken the public's faith in the validity of elections even further, playing into the other side's hands.