You have to be joking!!!
This is a sport where the ball is in play for a few seconds and then the game actually STOPS, which means the players effectively receive some rest until the next play.
Seriously, i'm gobsmacked anyone would think of them as the best athletes, it literally makes no sense whatsoever.
How on earth can nfl players in general be better athletes than Tennis pros, cyclists[Tour de France type] and pro boxers? Think about the fitness levels and athletic abilities needed for those sports when done at the highest levels, there is simply no comparison, without any doubt at all.
As far as football codes go when there have been comparisons and tests done with players from Football,[soccer] League, Union, and AFL,[aussie rules] the AFL players won with a good margin from the others.
Some players from these codes regularly end up running miles throughout the games they play.
NFL players are only actually moving for 11 minutes on average during an NFL game. Only 11 minutes of action! The rest is mostly standing around, with the occasional momentum-killing penalty review thrown in for good measure.
Yes there is more to elite athletes than just running but aerobic ability is vital.
https://gizmodo.com/5992583/how-far-do-you-run-in-different-sports