It is no more valid than saying “The Smith's divorce was not about physical abuse, because Mr. Smith promised to quit beating his wife.” If Mrs. Smith does not trust her husband, and does not believe he will actually stop the violence, then she will go through with the divorce despite assurances that the violence will end.
The Fire Eaters of the South did not trust the Northern politicians, particularly Lincoln. They did not believe that the abolitionist movement would end. They feared efforts to enact state by state prohibitions outside of any federal amendment. Finally, they wanted to expand slavery and the Kansas–Nebraska Act worked against that goal.
You have proved nothing.
I think rather that DL’s main argument is that the proposed Corwin Amendment proves the North was not anti-slavery. I have witnessed the evolution of his thoughts about the proposed Amendment. Firstly he came onto one of these threads proclaiming that Lincoln (by the proposed amendment) intended to make Slavery “express and irrevocable”! He was then shown that Lincoln meant that he saw no reason that the amendment shouldn’t be made express and irrevocable (not Slavery). Then he came up with (with zero proof) and pushed the idea on a thread here that Lincoln was involved in the writing of the amendment! Lincoln, upon bringing up the matter of the Corwin Amendment in his 1st inaugural, stated “which I have not seen”. But DL doesn’t, couldn’t believe Honest Abe’s own words. Instead he has to make them into pretend things that are more palatable to himself.
Everyone knows that Lincoln had already said to Greeley that if he could preserve the Union half slave, all slave or no slaves, he would do it. His primary objective was to preserve the Union! So there was nothing new in the proposed Amendment. Although DL seems to think there is.
Of course the “proposed” Corwin Amendment was only ever ratified by 5 States, two of which later rescinded their ratifications. One as recently as 2014. Buchanan actually signed it even though his signature wasn’t a requirement. So, not only is the “proposed” Amendment academic, it is a moot point. Despite DL handwringing it to death.