"A few decades ago there was a more popular genre of cookbook: a huge 200p book"

"Joy of Cooking" ©1975 is 915 pages. "The New Book of Middle Eastern Food" ©2000 is 513 pages. "Vegetarian Cooking for Everyone" ©1997 is 742 pages. "The River Cottage Meat Book" ©2007 is 543 pages. "Mastering the Art of French Cooking" ©2001 (40th Anniversary Edition) is ~720 pages, but this is just the first volume; the 2nd which is similar in length is an extension, true, but also illuminates uncovered areas in V1 like charcuterie.

I've got many more fat cookbooks, of which quite a few spend a 100 pages or more on basic techniques for the culinary area they illuminate.

I am not sure I would trust the parent commenter's accuracy on well, anything, really, if the data they cite are so disconnected from reality.