It's a good general rule, but it gets repeated as an absolute truth and that misses some detail. In my experience, some species really don't mind being a bit deeper, and I've seen people expose way too much flare, damaging the important surface roots. The real key is just not burying the main trunk in soil, but the perfect amount of flare showing can vary.
Honestly, my buddy went crazy with the "expose the flare" rule like @the_joseph said. He dug out so much dirt he actually cut through a bunch of the good surface roots on his maple. That tree struggled for two whole seasons before it bounced back.