Nah. Today I reached the chapter and it talks about for example, if you have 4 multiple choice questions, choose the one which has the most similarities to other questions. Why? Because questions are designed to have misleading options for answers, such that the answers will have some elements of the right answer in them to mislead you, making it quite unlikely that an answer which is radically different from all the others is the right answer, and making it more likely than average (randomly guessing, 1/4) that an answer which is similar to others is the right answer.

So if one answer is x^2+y/sqr(z) and another is 15y/sqr(z) and a third is 15/xy/sqr(z), then they suggest c because you have the most elements in common with the other answers, 15, x, y/sqr(z). This is literally in a McGraw-Hill (one of the 3 big billion-dollar education publishers) book on the GMAT.

It has nothing to do with any real world, practical application, and it's completely different from what you describe which are completely sensible approximation techniques. These aren't approximation techniques, they're guessing techniques and it's total bs. It's not teaching you a strategy to learning, to approaching problems to find meaningful solutions, or to approximate answers to questions, it's teaching you purely to game a test and the real world doesn't work like that.