The quote, by its very nature, is a reference to a future event that had not yet happened. He anticipated at the time of that future event, if it was to happen, that its participants would be states. Live with it.
Having them become states and leave the Union was never Jefferson's wish.
Apparently it was.
"The future inhabitants of the Atlantic & Missipi States will be our sons. We leave them in distinct but bordering establishments. We think we see their happiness in their union, & we wish it. Events may prove it otherwise; and if they see their interest in separation, why should we take side with our Atlantic rather than our Missipi descendants? It is the elder and the younger son differing. God bless them both, & keep them in union, if it be for their good, but separate them, if it be better."
He also apparently saw any split to be an exit from the union and explicitly used that term as well.
They were not in the UNION in 1803 thus, it was not a reference to splitting the UNION.
Your inductions are continually slothful. Jefferson EXPLICITLY stated that at the future anticipated time in which the split may occur, those territories would be both STATES and in the UNION. That is why he said "keep them in union, if it be for their good, but separate them, if it be better." It takes a person of either apalling dishonesty or utter stupidity to claim that a quote saying "keep them in union, if it be for their good, but separate them, if it be better" is "not a reference to splitting the UNION." You are evidently such a person.
They could have become separate countries had Congress allowed that but not by unilateral declarations without the consent of Congress. Jefferson did not maintain otherwise
Jefferson maintained that it was their call to make: "[I]f they see their interest in separation"