I disagree strongly. What Mr. Pournelle seems to be advocating is not a Melting Pot, but a Smelting Pot, where all becomes uniform and that which is not American is discarded. That is not what America was or is. He says, "You could study to become an American, and provided you did so, you would be accepted." No, this is inaccurate. All the complaints that exist now about immigrants not assimilating into the culture, not speaking English, forming their own communities, these are complaints that have existed against immigrants for at least a hundred years if not two. That's one reason why many big cities have a "Chinatown" and/or a "Little Italy", places like that.
And no, E Pluribus Unum does not mean "from many we become one." It means "out of many, one." A more modern translation, as best as my research can discover, might be "many uniting as one." In other words, many come together as one, not that many become one. One strength America has had has been the diversity of its people. And the notion of people sinking into a uniform culture is one I associate with totalitarianism, not with a free republic. So I cannot agree with Mr. Pournelle.