He's not saying that you need to ignore Fluff, he's saying that if they're in conflict, mechanics usually wins simply because it's easier to mold fluff around a mechanic rather than molding mechanics around fluff.
That's not to say that you shouldn't look to mold mechanics to fit fluff when you can, of course. A good mechanic that's backed up with good fluff is far superior to a good mechanic backed up by bad fluff. It's only when you can't get the two to work well together, that you need to sacrifice one for the other. Having to do so essentially means you've failed as a designer, but it's still better to have good mechanics and bad fluff than it is to have bad mechanics and good fluff.