The topic says it all but to be very specific: while the flag withing Stormwind Embassy for Mechagnomes has changed to show that you only need to complete the achievement “The Mechagon Threat”, the Stormwind NPC (I think Alysa Cloudspeaker - a Pandarian female) does not give the quest to actually unlock the Mechagnomes.
While I was standing in the embassy trying to work this out, another player who was doing the Rustbolt rep grind to unlock mounts gained the achievement for finishing the Mechagnome quest and unlocking them. I asked them about it and I was told that they had looked after the pre-patch but the quest did not show up until they gained Exalted rep with the Rustbolt (Mechagnome) faction. I unlocked the other allied alternate races that used to be locked behind a reputation grind without issue (all of them besides Void Elves which I’d unlocked previously). I did not and do not have exalted with any of their factions on any character, so they have actually removed the rep-grind requirement for all other Alliance unlockable races.