Completed this achievement on my Night Elf Druid last week. After completion I went to check if it was fully account-wide (thus granting my horde characters the option to claim a lost tabard), but it wasn't. I contacted a GM about it and they told me that there are two separate achievements for horde and alliance.
Does anyone know if faction changing the character I completed the alliance version on will then grant all my horde characters the option to claim the tabard?
Luckily I waited until the patch, and it seems now all my characters are able to get this tabard. So for anyone who had the same issues as me either as horde or alliance, go and redeem your tabards in case it is isn't intended (like with the Blood Knight Tabard).