It isn’t clear whether this is actually meant as a statement of ethical relativism. That would only be so if we add the further claim that each such “scale” or “pick” is equally legitimate. That is, if there are no reasons for making some choices rather than others. While it’s easy to read this assumption into the quoted statement, it’s important to note that this actually goes beyond what’s explicitly said.

So the most charitable reading might simply be to say that Caplan was less clear than he should’ve been. Really, I take it, he was saying that rather than worrying about weighing conflicting values, we should simply focus on the ethically most important feature, whatever that may be (perhaps suffering, the environment, or – far less plausibly! – taste).

So, I think the advice is useful as long as it is taken to mean ‘if you are going to worry, just figure out why these things are an issue for you and so sort out how they fit together and so how important they are for you.’ That way, it helps people to make these choices more easily in the future.

On the other hand, I would be worried if he actually endorsed choosing whatever you wanted, as a matter of his personal ethics. So as practical advice it’s good (no point moralising if you just want to help people here) but as ethical advice it is not.