The fact is that all charges of treason were eventually dropped, even in cases where it would unequivocally apply: so-called Copperheads in Union states, still US citizens, who provided aid and comfort to the Confederacy which had declared, and was making war against the US.
Lincoln had set the tone, and other leaders like President Johnson and US Grant followed up, to prevent the worst punishments of Confederate leaders.
So the argument today makes no sense, since people of that time carefully considered the question and decided not to prosecute -- certainly for political reasons, not due to somebody's scholarly legal opinions.
I know. Therefore I will “Grant” you that one. Grant went after the POTUS when he tried to pull that stuff with Lee.