I think you're arguing against something the grandparent didn't claim? They didn't say that various moral systems wouldn't have different outcomes, just that no

I think you're arguing against something the grandparent didn't claim? They didn't say that various moral systems wouldn't have different outcomes, just that none of them would be intrinsically correct. We can note, for instance, that "produces a thriving society" is a thing for which you're imposing a preference. There's no inherent reason why that's the true system, just that we think it'd be nifty if that was an outcome.