The Outside Forces rule states: "Whilst the challenge is ongoing, other models locked in combat can only allocate Wounds to the models involved in that challenge after all other enemy models that are locked in that combat (if any) have been removed as casualties, even if the models fighting in a challenge are the closest models."

So what if one of the units started the assault was only one model and is challenged (it must accept it due to Heroic Stand), do the other models of the challenger's unit get to allocate wounds on the single model that was challenged? The Outside Forces rule seems to only address "other models locked in combat." Does the "(if any)" part of the rules cover this?

So what happens in the case of challenging a unit of only one model? Do all the models in the challenging unit get to fight against the single unit while the challenge is ongoing?