The Achievement under PvP > Grizzled Veteran is unobtainable for Alliance Players because ‘Riding the Red Rocket’ isn’t a part of the physical game anymore.
The quest in itself got removed from the game, however the requierement is still in the Achievement.
Do you happen to have a screenshot? I’m currently checking for the last month every single day.
The reason for my doubting also lies in the fact that Wowhead gets their data by API-mining. A reliable way for Blizzard to let 3rd parties make AddOns or in this case a Database website.
They have mined the most recent cliënt, hence the game cliënt version being listed on their page.
So, if you can provide some backing than at least I have a starting point to fix this problem.
PS: I have added a bug report ingame and a friend of mine stated that she got the achievement without doing the Red Riding quest. So there is some dubious and very contradicting information I get left and right which doesn’t really help.