I would have to agree with you. Killed plenty on slopes that steep.
I read a very long study about a year ago that detailed bedding areas and how elk preferred to bed on a slope closer to 15% IIRC. They used GPS collars to show where they bedded and fed over the course of several years. I would say that is generally the rule I look for when finding bedding areas. But I also know of long-established bedding areas with beds dug out where they will stay all day long that are on those really steep hillsides. It gives them a great view and is normally near multiple escape routes. Similar to how the mature Mule deer bed in a place that is tough to get to and even tougher to predict their route of escape.
I wouldn't mind getting one of those books. Be pretty interesting to see how it compares to my own experiences.