This happened to me too after going through the asura starting area; I didn’t think much of it when I got the achievement chest other than “Wow, I must have been closer than I thought” and opened it. I rushed off to use a make-over kit on my new character and only then thought to check exactly what achievement had pushed me over. Now I see that I’ve now unlocked nearly every HoM achievement—despite not having a GW1 account.
I tried to report it in game, but the bug interface….