people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out,

The post is clearly referring to obesity and physical fitness. Average bodyweight, BMI, and obesity rates have risen rather dramatically since 1900.

You're completely ignoring the context of the statement. With specific respect to fatness, demographics have changed very dramatically very fast, which suggests a non-genetic cause. Average number of lungs had not changed all that much, which suggests a genetic cause, but no one even pretended to challenge that.