Is this undeniable proof that Blizzard is playing favorites with the two factions? I don't think so. Breavis of Nazgrel pointed out that Thrall is known as the chosen son, so the symbol could represent him rather than the entirety of the Horde. The battle will likely rage on about which side Blizzard prefers. Drysc has pointed out in the forums that your opinion on which side is favored is usually based on whichever side you do not play.
Do you believe this to be a coincidence or not?