I think the intructor wants to use the feminine simply to show that there would be gender/number agreement; if the instructor had used the masculine (simple neutral past participle, no agreement necessary), then you would never know if such a structure requires agreement.

I agree with William above in the string here.  I often encounter instances where some very good online French instructors give "agreement" examples in the masculine singular, leaving it totally unclear or ambiguous whether agreement is mandatory.  I think all examples should avoid that and use plural or feminine instead, so we can see the agreement.  For example, and I hope I get this right, "He broke his arm."   Il s'est cassé le bras.  There are two problems with this example:  two genders are ignored.  One, the subject might be non-masculine, and two, the object (here bras) might be non-masculine.  So ... we'd never know what to do with Elle s'est cassé(e) le bras, or, even worse, Elle s'est cassé(e) la jambe.  Only if you are a good student of orthography would you know that the proper versions are:  Elle s'est cassé le bras, and Elle s'est cassé la jambe.  Why?  I think I am right -- the strict rule states that there is past participle agreement with objects only if the DIRECT OBJECT precedes the past participle.  And in this case the bras and the jambe do NOT precede.  Also, the "s" in s'est is an INDIRECT object, not a direct object.  You should try all these examples in Reverso or Deepl or Google translate and see what you get -- they may include errors, unfortunately. 

Responding to William's comment in Chris' thread: l know of a rule which states that if you know the exact body part affected, the endings don't change. E.g., Je me suis brûlée  versus Je me suis brûlé le genou; Elle s'est blessée versus Elle s'est blessé au coude;  Elle s'est cassé la jambe, ils se sont coupé les cheveux etc. (A tip from Duolingo, l think).