In the original post from the OP he was referring to an outline -- not a book. Objectively it may well be quicker (I'm not sure easier or better) to write a basic outline for a nonfiction book than for a fiction book, especially if it's relatively bare-boned. For example, let's assume the subject is the basic systems of the human body. If the primary purpose of such a book is to write an outline that tells readers what these systems are, and the writer intends to use same to create a book, this is easy -- there's no creation involved. You find out what they are, you list them along with basic information, and you are done. So, if by nonfiction those who have posted are speaking about a formulaic book, a very basic reference work, and such, they may be right. However, if they are talking about writing an outline that represents a book that has a concept based upon the basic systems of the human body, I completely disagree with them.
If you want to write commercially acceptable, well received nonfiction, the gathering of detailed, factual info is a must. Some fiction writers do research, but usually far, far less. Nonfiction writers get an idea for a book and the first thing they should do is research the market to see if there's a similar book already in print, good or bad. If good, and the writer doesn't believe s/he can do better, they drop the idea. If bad, they do months of research, and only when they fully understand the subject, begin an outline of varying sorts. Now the nonfiction writer has to decide how he can write the best book ever written on the subject, include all the required information, and make it enjoyable and understandable, not a tomb. Commercial nonfiction is anything but bland.
Examples: Neal Bascomb, Simon Winchester, Jared Diamond, David Haldberstam, Max Frankel, to ame a few.