In case you don't want to go looking, here's the offending achievement.
Also, there is this one
so one would get stuck at 940/1000.
The NeoWin article above links to this
Major Nelson post with another good quote from Cierra McDonald, Xbox Achievements manager.
You can unlock an achievement at any time, be it on a game’s launch day or 3 years later. I guess you can say an achievement is like a promise in that sense. A challenge, on the other hand, is more like an opportunity – better grab it while you can! (emphasis hers)
XboxSupport
claims the achievement is not against official policy, but they are sometimes wrong.
I wonder if this can be obtained while offline. I know that Xbox One achievements can't be unlocked while offline. However, sometimes you can fulfill the requirements offline. Then when you reconnect and start the game again, the achievement unlocks. I've done this sometimes with KS Rivals so I can play against low level AI without losing my cloud save.
Regardless, that's just curiosity on my part. I also won't buy a game with such an achievement.