That's the point, though - he was making an intentional strawman.
An open-ended question: are our moral intuitions _as leftists_ (against capitalism, &c., not merely against chattel slavery) primarily consequential of the MOP, or can they lay claim to transhistoricity on account of contradicting those MOP? (My guess is mostly the first, inasmuch as it's a meaningful question.)
An interesting historical fact: capitalism is full of people who object to capitalism as such; early modern chattel slavery was surrounded by people who objected to it as such; state socialist systems, again, full of people who objected to it as such, although perhaps more discreetly so. In antiquity nobody seems to have objected to slavery as such, although of course lots of people objected to their own condition of being slaves. Why? There's an intellectual tradition of blaming Christianity; although that _has_ to be overly Eurocentric, right?