Nonsense. Davis could have been tried, perhaps should have been tried, but between delays and Chief Justice Chase believing that the sooner the matter was dispensed with then the sooner the healing could begin the charges were eventually dropped on Fifth Amendment grounds. Within the year Johnson issued his final clemency proclamation and the matter was settled for all time. Had Davis been tried on treason charges he would have been convicted.
Whatever. Davis wasn’t tried because the feds didn’t want to have his views upheld in court. That is the bottom line. It is not like the 1860s era republicans were a benevolent lot and let Confederates off the hook out of the goodness of their hearts. The Confederate states were not trying to overthrow the government because that would be treason. They just wanted out of a union they voluntarily entered, like a divorce. It is only common sense that states should be able to leave a union they voluntarily created and many people believed that at the time.