The only times I’ve seen Jason Brennan say anything about metaethics, his takes have been nothing short of terrible. Look at this one:

Either moral realism or moral nihilism. Everything else is a joke. Morality is either real or bullshit. Every in-between theory ends up being a disguised form of one of these or is incoherent. 

How does one even begin to assess this? Morality is either “real” or “bullshit”? Well, doesn’t it matter what specifically it is that’s “real”? And are we obliged to think it is only “real” if it involve stance-independence? If so, why?

As far as other theories, it’s questionable whether they’re “in between” these, as though there’s a spectrum, but even if we grant that, other views are not disguised and they’re certainly not “incoherent.” Expressivists aren’t disguising what they think. Neither are relativists.