I haven't found that to be true at all. I feed mine before we go out and they will work for as long and as hard as necessary. A few kicks certainly won't put them off...if mine worried about a few kicks, hungry or not, then they probably wouldn't be the ferrets I would choose to work.
Yes, each to their own. But I do not go ferreting without having had breakfast and I have to do considerably less than the ferrets, so I don't expect them to work on an empty stomach either.
ive just taken a few of my young (around the 8 month mark) ferts on their first outing, took to it like a duc