Well, the book I read also emphasized that even if you had no clue, you still couldn't lose anything by guessing; on average it would just come out the same as if you left it blank, so you might as well give it a try.

But I can certainly understand why this is easier to get in the context of a 4- or 5-answer question than 2. To understand the true/false case, you need to understand at least a little about calibrated probabilities.