What difference does that make? Permitting the possibility of future states seceding is no different from permitting the possibility of current ones doing the same, hence his explicit statement that leaving would involve a "separation" from the "union." Once again you are simply being slothful since you do not like what the quote says.
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.