But if the aim of our dialogue is true claims about reality, then you've got to talk about things that exist - whether the subject matter is 'oughts' or not.

Which would mean either that mathematical knowledge is false, or that there is a Platonic word of mathematical objects for it to correspond to.

OTOH, one could just adopt the Dogma of Empiricism that there is analytical truth which is neither 'about' physical realitty nor 'about' about any metaphysical one ( and that mathematical truth is anayltical). (and that mathematical truth is anayltical).

And if it is an analytical truth that, for instance, that you should do as you would be done by, then that is still applicable to real world situations by fulling "as you would be done by" for your own case.