That’s a completely wrong and stupid definition of relativism. Either because the book is wrong, or (and I’ll grant this is a possibility, because it happens a lot and it’s why the whole field of philosophy should be killed with fire) the ivory tower academic definition has gone so far beyond reality that it’s just completely absurd, and can be safely ignored as the ravings of a lunatic.
Either way, it’s a straw man. Just in the 2nd case, there’s a small group of batshit insane losers who actually believe the straw man is a real man, and they talk with it and have tea parties and shit. And get offended when you point out that it’s made of straw and they should get mental help.