What effect does Coat of Arms have on face-down creatures?
From that same page (emphasis mine):


  Creatures turned face down by Ixidron are 2/2 creatures with no text, no name, no subtypes, no expansion symbol, and no mana cost.


"Creature type" is a shorthand for a creature-specific subtype (205.3m in the Comprehensive Rules). Face-down creatures have no creature type, so Coat of Arms does not affect them.