Or more directly, they're healthy enough to bike to work.
I agree that exercise contains huge health benefits, but there's also a huge selection bias at work. Seriously unhealthy people probably can't handle the rigours of cycling to work.
Usually if you read the study you will find that they have already figured this out and measure it, these kind of simple objections are often accounted for.
My bad, from reading the parent comment I assume this was just reporting the correlation, and made no attempt to adjust for confounders. But glancing over the actual study they did (though there's always the question of how effected the correction was).
Beside that the percentage of people that can handle a 20-60 minute bicycle commute is so large that it doesn't really matter, everyone I know can handle it with a few exceptional exceptions.
I'm assuming a lot of stuff about this discussion though, I've not read the study nor the article, but similar papers on the subject usually have this in their models so...
I'm not so certain about that. There's about 40 people in my workplace, which skews fairly young. I can think of at least 3 whom would be seriously challenged by such a commute, mostly due to obesity.