If you're simply reading to inform yourself about a topic, it may be worth reading a couple of book reviews, and listening to an interview or two, rather than invest the significant amount of time necessary to really engage with the book.
A few hours of skimming and reviews/interviews may get you to 25% understanding and retention, which in many cases may be more than enough for your needs of being basically informed on the topic. Compared to the 50 - 100 hours necessary for a deep, analytical engagement with the text, that would only get you to 50% understanding and retention.
I like your summary of the article but disagree with the author. If one is measuring the "understanding percentage" of a book, I think he's actually looking for short-form information or answers (like gathered from a book review or blog post) rather than a book. For me, the value of the best books comes from the inspiration and impulse for original thought it gives. Not everything in a book is valuable for everyone, nor should it be.
For textbooks, of course cutting the detail gives you an overview more quickly, but it's just an overview. And this entire point doesn't apply to fiction books.
Maybe it's a question of what type of understanding we want from each book. Sometimes that's answers, sometimes that's understanding, sometimes diversion or inspiration -- they're not all the same. Consulting reviews but not the book is an awesome shortcut to get answers, but then you're not reading the book, but the reviews.
I think the gap between thoughtful, slow reading and quick reading is much larger. Assuming of course that slow reading means thinking about the sentences, probably highlighting passages and scribbling a few notes.
Doesn't apply to every book of course, and depends on the aspect of understanding you want (your own thoughts or what the author meant).