Yeah. Stewart seems like one of the old school guys, like he would fit in back in 60s and 70s when they wernt considered athletes by any means.
But it seems like these days its well known how much they work out and Jimmie Johnson is one of the more famous gym rats. Coincidentally he has what the second most titles ever, or five in row or something?
Maybe they arent all athletes, but being athletic likely helps you perform better.
Correlation != causation. Kyle Busch wins a lot. I wouldn't say he's an out of shape slob, but he doesn't look particularly athletic.
Being athletic helps in a lot of aspects of life, including racing cars. However, you don't need to be an athlete to win, and not all NASCAR drivers are athletes.