Further, I suspect that these three categories may have varied from place to place or even time to time-- no one was working in the depths of winter, or during religious festivals. It almost seems like these categories are too broad to be a useful framework for understanding medieval society. Is this a reasonable conclusion? What would a more reasonable framework look like?
no one was working in the depths of winter, or during religious festivals.
You might be interested in these threads on peasants' work during the winter
Did peasants in eighteenth century France really "hibernate" during the Winter? slam-dunk debunking by /u/Talleyrayand
How did the people in Scandinavia live during the winter in the 12th century? answered by /u/sunagainstgold
and What would an English serf do to occupy his time during the long periods of darkness in the fall and winter? also by /u/sunagainstgold
Hi there. All your inferences are all very good ones; and largely correct! (Sure, I suppose there might have been more going on in medieval farms during the wintertime than you might think, but along broad lines your reasoning is right)
Your logic neatly aligns with something that Medievalists keep repeating here on r/AskHistorians:
Feudalism Didn't Exist.
You can read an AMA we had on the topic here. And I myself offered a cursory examination of some ideas in this discussion. Of course, this shouldn't discourage additional posts by people who have more to add.
FWIW: Medieval Books of Hours often contained illustrated 'Labours of the Months' to show what might be a suitable activity at any given point of the year. According to these, Midwinter might be a good opportunity for a farmer to be making household repairs, mending clothing and tools or to be weaving baskets and other wickerworks.
That aside, you're correct in saying that the author was making a very glib generalisation which omits prominent sections of society. As early as the 7th Century, mercantile and maritime trade constituted major economic activity around the coasts of the English Channel. By the tenth century, trading ports like Bristol, Chester and London (diminished somewhat due to the significant impact of a major plague) were some of the most rapidly expanding and lucrative settlements in England. In the 12th Century, a silver boom in Carlisle made that town one of the most attractive trade destinations in Northern Europe. Obviously, merchants and traders remained a minority among the overwhelmingly agrarian population, but then again, so did the nobility.
The model also overlooks a not insignificant amount of overlap. The majority of 'those who pray', for example, would have been monks who, between their duties and depending on their order, would have responsible for tending the often substantial estates attached to their houses, albeit alongside 'lay brothers' or tenant farmers. While the pop-history idea of waves of untrained peasants being dragged off to war by their feudal lord is far from accurate, organisations like the Anglo-Saxon fyrd were effectively a trained militia recruited from the peasantry, who would be called up to defend their homes in times of crisis.