I think it’s as the commentary says, I believe the Buddha, Kassapa and Mongallana are just experiencing the first dukkha of suffering suffering.
They have no corresponding mental suffering due to impermanence (of health), but nonetheless, it’s not convenient to be in sickness still even without mental suffering. Thus they needed someone to recite the 7 factors of awakening for them to rejoice and body heals. This indicates conditionality, so in that sense, they are subject to suffering inherent in conditionality for needing to go through that process to heal instead of immediately heal.
This reading makes it consistent with the rendering above in my initial posting.
I stick to my original post too, on your notion that the 3 dukkhas are eliminated by some level of sainthood, I don’t think so.
Stream winners still have roots of greed and hatred, thus they are subject to mental suffering of heartbreak due to divorce etc. That’s clearly under suffering due to impermanence and suffering suffering.