AsiaMiles has the same wording for getting miles according to the class purchased rather than that travelled, but you certainly get it credited as First Class if you travel in A on AA domestic using a DONEx. I think since the rules specify explicitly that you book into A on these flights then that is considered as the Fare Purchased for those segments.

On the other hand, if you use an AONEn on an AA route that doesn't offer First then you only get business mileage credited.