The author here makes a good point until he asks his final question. In the case of Dr. Martin Luther King I don't believe copyright would have had any influence on the likelihood of delivering it. The purpose of the speech was selfless and so despite the copyright issue being disappointing it doesn't seem like he would have said "well, since future students won't be able to read the full text of my speech why deliver it?" Though he understood the significance of the march and probably knew his speech was important is there any evidence to suggest he knew how iconic it would become? And even if he did, was he going to forego addressing his audience, the people assembled at the Lincoln Memorial, to make a statement on copyright? It's doubtful. His speech was for the audience there. The fact that it could be reprinted and studied by future generations is just a bonus. The idea that King would be less likely to deliver his speech because of copyright is not realistic.
That said, this also isn't a convincing argument for the abolition of copyright either. I don't know if that was the point though. What this does show is how a copyright can serve the opposite of its intended purpose and hurt society. But for every case like the King speech there are plenty more that are examples of its benefit.
It's disappointing that we do not have free and open access to the I Have a Dream speech but its not a good argument against copyright. Copyright is still an issue that boils down to its use and has to be considered on a case by case basis. I don't think it ever has been all good or all bad and overlooking the vast sea of nuance there doesn't help proponents or opponents of it.