I agree completely–as well as pointing out that any successful secession gives us two points of view for treason–that of the former nation, and that of the new nation.
Also, we need to be careful not to confuse the ability to achieve factual independence, which is a question of politics or military power, with the legal right to independence under the laws of the former nation. That’s why I think we should try to be careful not to just use the definition of secession you point to, which is how many people understand it today,
without being clear if we mean simply getting away, or getting away “legally.” (as I posted earlier, James Buchanan would call the first revolution and the second secession).