You do know the difference between knowing a bit about a certain school of magic, and being a master of it? It's safe to assume mages learn a bit of every school during mage training, but like the book says, they only opt to specialize in one, maybe two. Considering the profession's skill with the enchanting school of magic is more versatile than the average mage's skill with it, I think it's safe to assume that 'enchanters' are those who specialize in the enchanting school of magic, while the other mages (including player mages) don't.
- - - Updated - - -
You don't disenchant weapons as a Death Knight. The weapon's original magic is unaffected. Lore-wise, you simply engrave runes upon the blade, from which you use to power your abilities (lore-wise, it seems a DK cannot use their abilities without it).
Probably because they're not the same thing, and are barely (if at all) related?And you are correct... Runeforging interferes in no way with the Enchanting profession.