Yes, I have also heard about the idea that it would be a staff-slinger. I don't have my Head's AMPW available right now, but if I remember it correctly the staff part was not based on any historical sources, but instead he thought it un-likely to shoot darts without a staff. There are a few slingers that have made hand slings and used them to shoot darts without problems, so I do not think that the staff is necessary. (But that is not just my thoughts; there are a few articles which support the idea that the kestros had no staff).