It depends on what you mean by "valid". If a criticism is correct, then it is "valid" in the technical sense, regardless of whether or not a counter-proposal was provided. But condemning one solution while failing to consider any others is a form of fallacious reasoning, called the Nirvana Fallacy: using the fact that a solution isn't perfect (because valid criticisms exist) to try to conclude that it's a bad solution.
In this case, the top-level commenter didn't consider how moral absolutes could be practically implemented in Claude, they just listed flaws in moral relativism. Believe it or not, moral philosophy is not a trivial field, and there is never a "perfect" solution. There will always be valid criticisms, so you have to fairly consider whether the alternatives would be any better.
In my opinion, having Anthropic unilaterally decide on a list of absolute morals that they force Claude to adhere to and get to impose on all of their users sounds far worse than having Claude be a moral realist. There is no list of absolute morals that everybody agrees to (yes, even obvious ones like "don't torture people". If people didn't disagree about these, they would never have occurred throughout history), so any list of absolute morals will necessarily involve imposing them on other people who disagree with them, which isn't something I personally think that we should strive for.