That's essentially what it is, despite courts nonetheless basing rulings on those mostly private (i.e. never publicly stated, and for a reason) statements. Jefferson argues in some of his other private correspondence about the Constitution in favor of both Nullification (the principle states can ignore locally unpopular federal laws within their borders) and Secession (actually, a LOT of the Founding Fathers made Constitutional arguments on the latter, usually starting with the colonies refusal to ratify the Constitution without a guarantee of secession rights). Funny how Jeffersons far more copious writing about those subjects don't have equal weight.

O'Donnell was right to sneer at the suggestion any of the words in the First Amendment enshrine separation of church and state in the Constitution. What we have in practice is a limited separation of church and state, which is pretty much what the explicit constitutional language requires, but we don't have anything like a pure or absolute separation or religious people, let along religious leaders, could not also be political leaders. To completely separate church and state in a self governing country would be to bar the religious from office (which is actually unconstitutional due to the section that bans religious tests for office holders). Once again, that's probably why the Constitution DOESN'T require separation of church and state, though it easily could have. Remember that the whole context of Jeffersons letter to the Baptists is how much government can dictate their religion, the very thing alarming O'Donnell and a lot of modern day Baptists when they see "separation of church and state" promoted as both an absolute and constitutional.