Respectfully disagree. It WAS about slavery - among other things. But slavery was first and foremost. You’re obviously correct about slavery not being prohibited by the constitution. If that were not the case there never would have been a union to begin with. But that in no way contradicts the statement that the civil war was fought over slavery.
If you want to make the argument that Lincoln and the north acted extra-legally and extra-constitutionally - no argument from me.
If you want to say that slavery was not the fundamental issue dividing the north and the south - from say 1840 to 1860 - then you’re welcome to your opinion but the facts say otherwise.
In order for Slavery not to be prohibited by the constitution the slaves must not be Human. Slavery was always prohibited by our founding document.