Orbiter does not model Oxygen content in an atmosphere, and so XR SCRAM performance depends only on atmospheric density and ship velocity. In any case, the Mars atmosphere is so thin that I wouldn't expect much thrust from the SCRAMs on Mars.
Surface pressures on Mars are equivalent to about 30-50 km on Earth, so really actually right in the middle of the SCRAM operating range (24-60 km from the checklist in the XR Operations manual).
And realism-wise, it's not even that bad for the SCRAMs to be able to operate on Mars. Assuming chemical propulsion (lower main engine ISP settings in the config file), the XR could be assumed to carry oxidizer for the SCRAMs on no-oxygen worlds, which would reduce fuel efficiency but still be better than rocket propulsion at equivalent exhaust velocity (because we're using atmospheric air for propellant). Assuming nuclear propulsion (higher main engine ISP settings), there's no need for the XR to carry SCRAM fuel at all, the incoming airstream can just be heated from the reactor, regardless of composition.