The studio is definitely very busy with their development in any case, as they have been expanding the lineup of the “Angry Birds” franchise, introducing new games that explore even more themes. They seem to have slowed down their releases somewhat lately, but we would expect to see more games coming out in the near future, as long as the brand name remains popular.
It’s also worth noting that the list of achievements released by XboxAchievements may not be final, as it’s possible that the studio has additional achievements prepared for the actual update. If the list that was discovered was work in progress, then we wouldn’t expect its contents to be finalized either.
Last but not least, there is also the small possibility that this was just a scrapped idea that isn’t going to make it into the final game, but this is not very likely given the timing of the discovery, as well as the rate at which players have been requesting the feature to be added to the game lately. Rovio know what they’re doing and they’re paying attention to their community.