The phrase has had a long afterlife, and most of it assumes more than the sentence says. What the chapter says is narrow and practical: the person who follows the seasons does not end up with the illness, so the work is done before there is anything to treat.

The closing image is deliberately undignified. Digging a well when you are thirsty, casting a spear when the fighting has already started — the point is not that treatment is useless but that it is too late.