I guess one could claim Lincoln and the Congress were doing all they could to "protect Southern slavery", if you ignore the fact that the Republican Party was totally opposed to any expansion of slavery.
I guess that's protection of a sort.
A more reasonable interpretation than DiLorenzo's is that Lincoln and the Congress were willing to tolerate slavery where it already existed, but they were adamant (at least Lincoln was) that slavery remain on a path to ultimate extinction.
Whatever Lincoln thought, it was vastly more advanced that what some southerners were saying:
It was because the free Negro menaced the institution, because manumission undermined it, because all self-help systems for the slave corroded It, that pro- slavery men urged new legislation. Their object was not to surround slavery with an atmosphere of terror. It was to shore up an institution built on quick- sand and battered bv all the forces of world sentiment and emergent industrialism.
Ruffin was personally the kindliest of masters. The unhappy fact was that it had become impossible to safeguard slavery without brutal violence to countless individuals; either the institution had to be given up, or the brutality committed.
The legislators of Louisiana and Arkansas, of Alabama and Georgia, with humane men like Ruffin and the Eastern Shore planters of Maryland, had faced this alternative. They had chosen the institution. The Richmond Examiner stated their choice in unflinching language:
It is all an hallucination to suppose that we are ever going to get rid of slavery, or that it will ever be desirable to do so. It is a thing that we cannot do without;that is righteous, profitable, and permanent, and that belongs to Southern society as inherently, intrinsically, and durably as the white race itself. Southern men should act as if the canopy of heaven were inscribed with a covenant, in letters of fire, that the negro is here, and here forever—is our property, and ours forever—is never to be emancipated—is to be kept hard at work and in rigid subjection all his days.
This has the ring of the Richmond publicist Fitzhugh, and would have been repudiated by many Southerners. But Jefferson Davis said, July 6, 1859, "There is not probably an intelligent mind among our own citizens who doubts either the moral or the legal right of the institution of African slavery." Senator A. G. ' Brown said September 4, 1858, that he wanted Cuban, Mexican, and Central American territory for slavery; "I would spread the blessings of slavery . . . to the uttermost ends of the earth." Such utterances treated slavery as permanent, and assumed that it must be defended at every point."
-- "The Coming Fury" by Bruce Catton
Nonsense. The Lincoln's little pro-slavery amendment would have had the effect of artificially extending the institution well beyond its likely life by making it near impossible to repeal at a future date. You can fib all you like about The Lincoln, Walt, but that won't make his amendment go away nor will it change the text of that amendment, which would have protected slavery indefinately.