US Core is silent on the binding for  AllergyIntolerance.reaction.substance.   In the base specification it is an example binding so the implementer may indeed choose to use the same codes.   Unless there is a consensus from the implementer community to define the  AllergyIntolerance.reaction.substance  element in US Core, US Core will remain silent on the element.  The commenter may wish to address both bindings and alignment with the HL7 patient care workgroup.