Why mark "nous sommes brossés" as almost correct?

Eric, I am not sure if you are questioning it because “nous nous sommes brossés les cheveux” is actually incorrect, or because you think it is correct ?

This question comes up often - there is no agreement of the reflexive verb because the direct object here is ‘les cheveux’ and follows the verb; the reflexive (object) pronoun ‘nous’ is an indirect object here, coming before the verb.

Céline has explained previously why the incorrect answer is being ‘accepted’ as nearly correct at present. 

See link:

https://progress.lawlessfrench.com/questions/view/conjugation-of-reflexive-verbs-in-passe-compose-nous

This particular exception is not covered in this particular lesson. The leson simply says:

 "Note also that the verb must agree with the gender and number of the person.

The lesson says nothing about direct objects. Is this covered in a different lesson? In this case, the question should be linked to that lesson, not to the lesson that doesn't mention it.