What I am about to post must be taken in the right context, I am in no way making any comparison to this level of athleticism or skil, but I want to add my little bit of experience to the argument as an opinion with at least something to back it up with.
Having played both AFL style football (not in the AFL) and having played gridiron (of course not in the NFL), but doing these together over a few years at local level I have to say that although it looks remarkable that he has made an NFL roster, it is not without some level of expectation.
he must have given himself a chance, and that is the key here, he gave it a chance and had a good old fashioned go at it.
Going back a way, I always though Garry Ablett snr would have been a great full back in the NFL, certainly his some Garry jnr could have performed as a running back, so too could have someone like Billy Slater in my opinion.
Yes, the sports are different, but if you have someone that good athletically/powerful and he has a passion with the drive to have a real crack at the NFL, I can see others down the track that could do it also.
Good for Jarrad and I hope he does get a decent run this season, his jersey's are the number one selling tops for the SF 9'ers right now, so I am loving the media interest, all positive to me...