Athene sat down with some of the guys who had helped Fs to 90, and chatted with them about how they'd done it. They asserted that it was simply related to a buff that was put onto them by some mobs in Townlong Steppes, shamans to be specific, which they'd used to farm mobs for XP. They said they'd done nothing else, but the fact that Blizzard rolled Fs back didn't really support them.
The latest development in the story happened late last night, when Blizzard Community Vaneras posted the following:
So it seems from this that, actually, the claims were true, there was no exploit, no cheating, and the achievement was legitimate. Fs's achievements have also been reinstated on the armory. Almost 7 months down the line, congratulations!