My understanding is that there is not just one Theravadin Abhidhamma, so maybe Dr. Gethin is correct (that sati is always kusala) according to some source. However, I have been reading Bhikkhu Bodhi’s translation of Anuruddha’s Abhidhamma Sangaha, and in there, sati is given as (exclusively) a universal factor for the beautiful (sobhana, not unwholesome) cittas, which are not all wholesome (kusala). In particular, the cittas of Arahants are sobhana but not kusala.
According to this same source, right mindfulness is a subcategory of mindfulness where the object of consciousness is within the four foundations, e.g. body as body, etc. So this source does not support the assertion that Theravadan Abhidhamma classifes all mindfulness as right mindfulness, even though it is all sobhana.
The paucity of English translations of more Abhidhamma texts makes it difficult for us linguistically-challenged folks to delve any deeper. In any case, this is a very complex subject, and I don’t find such long reactions surprising.