So, this criticism is implying that simply understanding human speech will (at a minimum) let the AI understand moral philosophy, which is not trivial.

I don't disagree with the other stuff you said. But I interpreted the criticism as "an AI told to 'do what humans want, not what they mean'" will have approximately the same effect as if you told a perfectly rational human being to do the same. So in the same way that I can instruct people with some success to "do what I mean", the same will work for AI too. It's just also true that this isn't a solution to FAI any more than it is with humans -- because morality is inconsistent, human beings are inherently unfriendly, etc...