The mistaken idea by the "constitutional scholar" Mr. Amar that without Lincoln the slaves would never have been freed or that we wouldn't have subsequent constitutional amendments banning slavery is preposterous. It's the old faulty logic that if one person hadn't done a certain thing, no one else would have ever done it.
IOW, you can’t prove a negative.
He doesn't say that. He doesn't speculate on whether slaves would have been freed without Lincoln. And he doesn't say that "we" wouldn't at some point have had anti-slavery and citizenship amendments without Lincoln.
But that "we" got them -- that we got the amendments we actually got when we got them -- does owe a lot to Lincoln. Look, I could say that without Washington or Jefferson we eventually would have become independent, but that doesn't change their role in making us independent, in the real world, rather than in some speculative counter-factual reality.
And why do I put "we" in quotes? Because it's entirely possible that in your counter-factual reality we wouldn't be one country, but several, and while slavery probably would have ceased to exist some time after the 1860s, racial equality could have taken a lot longer to come along.
This article is chump-bait. Elias Whathisname takes a scholarly book that may or may not have a valid thesis and tries to twist it into some salvo in today's culture wars and people respond emotionally based not on the arguments that may be in the book, but on the attitudes Elias attributes to it.