I think the idea of the initial post is that, when presenting someone with a "guide" for running a game, you kind of expect someone to have read the whole thing at least once, and then use it for reference.

With the case of the 5e DMG, it actually has quite a lot of good advice in it, but most people running games haven't read it fully... You constantly see complaints about 5e saying "there's no advice for (x)" where the advice is just in the DMG