“Elles ne l’ont pas fait exprès” does NOT follow the direct object rule, and the lesson states this clearly. Is this because this is a case of le/la referring to a concept, so it’s not a direct object? Could their be a sentence in which a direct object would be used, and therefore require agreement?
I believe that this is an instance of the neuter pronoun, 'il', which, as you say,can refer to a concept or an entity previously mentioned and since it is 'neuter' it has no influence on the past participle which remains invariable.
No other direct object pronoun fits this construction.
Of course in the passive it is entirely possible to have agreement with a preceding direct object:
Cette destruction gratuite que l' on voit a été faite exprès.
Hope this helps,
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