I see no reason to doubt the finger story. If I were in charge, I would tell the troops to collect fingers and throw 'em in a gunny sack.
Think about how much easier this is versus the sample collection protocol you mentioned. About all you need is a knife and a sack.
I'd be interested to know where and how they managed to obtain a KNOWN sample of OBL's DNA for use in the comparison. Does anyone know if such a proven sample exists?