I'd be willing to bet any amount of money that Jimmie Johnson would run circles around Donovan McNabb. Obviously not every NASCAR driver is like Jimmie, but every football player isn't Calvin Johnson, every basketball player isn't LeBron James, etc. The majority of NASCAR drivers are in very good shape. Even the ancient drivers like Mark Martin still work out every other day to try to remain competitive with the other drivers.
Also, I think McNabb would kill to look like this:
I don't even like NASCAR, but it was a laughable comment on McNabb's part.
An athlete is defined as someone that is proficient in sports and or physical fitness. NASCAR racing is a sport, and every race (not to mention the workouts leading up to races) is a physical workout. So they pass the athlete test for me.