The fact that peasants did not work more hours is tied to the fact that they were agrarian laborers. Once you've brought in the harvest, what is there to do but lie around and wait until the end of winter? Indeed, "lie around" is most of what they did, and remaining inactive to lower their metabolism was actually a survival strategy to deal with the limited amount of food they had: burn fewer calories, eat fewer calories.
As the article notes, peasant life consisted of working only part of the year, with longer workdays during harvest season, and short work days (or no work at all) during the rest of the year. In some climates, like the Pyrenees and the Alps, people essentially "hibernated" during the winter: they stayed indoors, lived off their food stores, and expended as little energy as possible.
Citing Graham Robb's The Discovery of France: 'An official report on the Nièvre in 1844 described Burgundian day-laborers thusly: '“After making the necessary repairs to their tools, these vigorous men will now spend their days in bed, packing their bodies tightly together in order to stay warm and eat less food. They weaken themselves deliberately.” Entering this inactive state was a physical and economic necessity. Lowering the metabolic rate prevented hunger from exhausting supplies.'
The time that they spent not working was not some leisurely edifying "vacation" that they enjoyed as a luxury; for French peasants in the 1700's and 1800's, being inactive was a survival strategy.
The parent article paints the time spent remaining inactive in a romantic light, remarking that "the medieval peasant took his sweet time eating meals, and the day often included time for an afternoon snooze." The reality is probably closer to the fact that he probably risked starvation if he regularly engaged in any sort of vigorous activity (no surplus calories to burn on frivolities like dancing or sports or other active forms of recreation), and it's quite possible that the afternoon naps may have felt more like an obligation than an opportunity to "relax." Lying around all day doing "nothing" sounds less like a vacation and more like a prison sentence (quite literally, in the sense that it probably resembles the life that some people live inside of modern prisons, doing essentially nothing as they wait for the days to pass).