I know how the Constitution defines treason. Secession is not treason, not even close. Confederates didn’t divide the nation. They LEFT it — the states that remained in the union were still united.
Secession was partly over slavery, but secession is not war so the Constitutional definition of treason does not apply to secession.
And the fighting did not start over slavery. Lincoln’s proclamation calling for volunteers to come South said nary a syllable about slaves, slavery or freeing. That was tacked on well into the fighting.