He is right, it is just a quick calculation - of course it is by no means accurate. It's just a little something to bring up with people when they ask. All people are different and for an accurate fit you should see a bootfitter. The only way to really know what size boot fits you is by measuring your foot and trying it on, duh.
And yes, you are probably right about your 28 = 11 theory but in essence it's the same as the 2+8 =10, but you're taking into account the fact that you should buy your boots snug. You're just going into more depth than he is, big whoop - not enough to make such a huge deal out of it, stop hating.
He can't "fail" because he doesn't mention bootfitting. It's generally considered common knowledge that bootfitting is the best way to go, so I don't see how that makes any sense.
Anyways, as a ski shop employee and a bootfitter, ghettochump or whoever should go to a shop and buy boots. At least then he'll have a boot that fits and it'll be much harder for him to steal them.