Partial Reprieve
The proposed moon does not NEED to be in an inclined orbit. It could even have been equatorial. After the impact, Mars' surface would have slowly rotated to align the spin axis with the large crater, just as the Tharsis bulge is thought to have migrated nearer to the equator.
OTOH, it's unlikely that the moon would have remained intact as it spiraled-in, so a non-orbiting body is still the preferred candidate for the impactor.