Secession was not thought to be unconstitutional until the concept was challenged by the Lincoln administration.
Ergo, if the concept was thought to not be illegal under the then current constitution it would follow that it couldn't be treasonous and would not be referred to as such by those in authority.
It simply isn't true that nobody thought that secession was unconstitutional before Lincoln. James Buchanan said as much in his December 1860 State of the Union message.
According to Buchanan, unilateral secession was only justified as a revolutionary act of resistance to tyranny, and not under any other circumstances. That was a widespread understanding at the time.
Since the US wasn't a tyranny in 1860 unilateral state secession was unconstitutional. Buchanan, though, didn't think the federal government could do anything about this unconstitutional act.
There was a widespread understanding in 1861 that the country was faced with a revolutionary situation. This affected Lincoln's thinking and actions, and Davis's as well. Davis and other secessionists were acting outside and against the law more than Lincoln was.
It was thought to be treasonous.
"No man, no association of men, no state or set of states has a right to withdraw itself from this Union, of its own accord. The same power which knit us together, can only unknit. The same formality, which forged the links of the Union, is necessary to dissolve it. The majority of States which form the Union must consent to the withdrawal of any one branch of it. Until that consent has been obtained, any attempt to dissolve the Union, or obstruct the efficacy of its constitutional laws, is Treason--Treason to all intents and purposes. . . . This illustrious Union, which has been cemented by the blood of our forefathers, the pride of America and the wonder of the world must not be tamely sacrificed to the heated brains or the aspiring hearts of a few malcontents. The Union must be saved, when any one shall dare to assail it." [Richmond Enquirer, 1 November 1814--The Enquirer, by the way, was Thomas Jefferson's favorite newspaper).
You might also look into what happened to the Federalist party and the accusations that drove them to extinction.