The difference is that he was speaking of new formations from the territory purchased which might never even be admitted as states in the first place. Thus, it would have been perfectly constitutional had Arkansas organized as a territory and petitioned Congress to be allowed to form a separate nation and Congress agreed.
But to attempt to destroy and organic unity such as was created by the Union is like taking an arm off a body.
You're being slothful again, fakeit. Jefferson specifically talks of their departure from the UNION, mentioning the word union by name. He gives absolutely no reason whatsoever to support your bizarre claim that he was talking about anything other than future states.