It's not that he's not physically capable. It's that he's built for an attrition sport, rather than an explosive one like the NFL. A year of training should get him in shape for that no problem.
Yes, he could learn the plays and intricacies of the sport too.
But the main reason why people doubt he'll make it, just like with Jarryd, is that why would any franchise take a risk on someone who has never had anything to do with the sport? When they could sign one of the hundred free agents who have played it, or draft a college player? All of whom are as much of an athlete as Holmes.
I hope he succeeds.