Hi Perkele, Vincent,
Thanks for the quotation, Vincent. As I said, Ven Sujato only mentioned in passing that there was disagreement between sects over the possibility of an arahant "falling from deliverence". This is consistent with your quotation, which seems to imply that some
arahants are "safe" and others are not.
Ven Sujato specifically mentioned the Sarvastivadins as one of the sects that held a different view from the Theravada regarding Arahants.
I have no further information on the subject, but I think that it is important to recognise that the suttas of different sects evolved over time and some of the changes may be a result of doctrinal arguments. I.e. the suttas may have been changed to support the doctrine of the sect. It's not so black and white that the derived doctrine is in the commentaries and abhidhamma and the suttas are untouched.