I don't think he was implying that "the U.S.", as a singular entity, views the Second Amendment as immutable. Rather, I think it's a reference to the fact that numerous people (on this forum and elsewhere) view the Second Amendment as unassailable (i.e. "It's a right; get over it.").
There's a few ways to look at it. For example:
I think that self defense is an inalienable human right, and that it includes the right to keep and bear arms for that purpose. I think this right should be recognized and protected by the laws of any democratic society. That's generally speaking, in the context of human rights, how I look at it.
In the context of the Second Amendment, I see it as being clearly codified as a right. In US law and governance, this means certain requirements must be met, in order for the government to infringe upon it. The existence of the Second Amendment can't simply be ignored by those who wish to impose stricter firearms ownership regulations on Americans. Regardless of my position on gun ownership as a human right, it clearly has some protection in US law. "It's a right, get over it," so to speak.
In the context of the US Constitution as a whole: The document codifies the boundaries of government intrusion into the lives of its citizens. It is the basis for all other laws, and the standard by which government authority is judged, in America. This is why its clauses and amendments cannot simply be ignored by those who wish to make improvements. Any such improvements must be made within the constraints of the law of the land, or else those constraints must be altered first. In the US, this means either...
.... Regulating within the boundaries of the Constitution as written, and as interpreted by the high court, or...
... Amending the Constitution to permit the desired regulation.
Again, "it's a right, get over it".
I think that all three of these views are reasonable views. I can understand some disagreement over the first view: The right to bear arms as part of the right to self defense. I can't really understand any disagreement over the second two views: That the regulations imposed by the government of any nation should acknowledge and abide by the laws of that nation, according to the rule of law practiced by that nation.
In the US, I think that obviously means that any regulation that does not pass constitutional muster has to fail for that reason. Any regulation based on the premise that gun ownership is not a right must first remove the codification of that right from the foundational document of US law.
Am I missing something? Are gun control advocates seriously frustrated by the fact that they have to deal with the Constitution as it stands today, rather than simply ignoring the bits that interfere with their idea of a Better Tomorrow?