I think you're misunderstanding. The point isn't about the ability to infer objective truth from a dissembling source, nor about how to prove an objective lie from that source.

It's about cultivating an ability to detect when you are probably being lied to by an otherwise authoritative source. And it's something americans have traditionally been very bad at.

(Including, I have to be honest, you. In fact it was reported just last week that Trump was sicker than the public was informed and that his blood oxygen did indeed drop into the low 80's. Your desire to believe the authority figures you trust led you to buy the spin. But people with this kind of "Autoritarian Muscle Memory" described were clued in much earlier.)