Having known a few, and having watched numerous programs involving BUD/s training, I can say that all SEAL-candidates are super-fit, but few are 'freakish' in height or weight categories. So I would argue that most would not be Div I/NFL. These guys would be Div II football, no problem. Given the above average intellect required I'd say Ivy League footballers would be a more apt description. But even that misses the point.
These guys can swim or run all day long... until they break down. That requires the kind of mental toughness required of a tri-athlete.
"Mental toughness is not jumping on a live grenade to save your family, friends or colleagues. Mental toughness is throwing your daughter on the grenade because she is of the least use for the survival of the larger group."
Physically fit goes with out saying. The human body can be shaped and trained and all that. But the mental toughness goes way beyond triathlete or anything most will ever be exposed to or comprehend.
I don't speak from direct experience per say. And those in the elite programs need no ego stroke. But there is a level of mental toughness, determination, passion and dedication to self and team that cannot be taught. There is a place in some individuals where a very simple thing becomes worth your life without question or hesitation; Where the measure of your own self worth is loyal and expendable component of a team. You worry more about what the team will think of you if you die (fail) than whether or not you survive. Failure truly becomes "not an option".
Think of driver ants or bees defending their nest with regards to commitment. Then remove all ideas about what defenses there are to deter those nuisances. Now you are approaching the mental toughness that will compel a human to perform better than is anatomically possible.
Mental toughness is not to be compared to that "hero moment" that most of us are capable of. Mental toughness is the 96th day in a row of pushing past the physical limits of your body into virtual torture mode on shear will power, all while having the option to quit with honor. It is also failing 87 times before succeeding while having no belief in failure.