This claim is like being a "little bit pregnant."
You either are or you are not.
The Corwin Amendment protected slavery to just as great of an extent as the Confederate (and the US Constitution) did.
There is no half measure when it comes to keeping slavery legal, and the Great Liars of the North would have us believe that all their representatives voted to make slavery permanently legal, but later fought a war to end slavery.
Of course it did.