So RE2 still lists this hidden achievement. If this was a mess-up it would likely have been resolved by now after all the media attention.
With RE3 likely to be revealed at the SOP next week, it's possible this might be related to a RE3 demo or some other update related to RE3.
I bring this up because datamining for Capcom titles has been an issue for Capcom in the past. But depending on the implementation of said data as a "update" to an existing game could possibly help negate that to an extent. While it might be odd to have achievements related to a demo, the one shot demo for RE2 on Steam had achievements as well.
Just something to think about as we all wait patiently for more news. Since whether it's a demo or not, SOMETHING is coming to RE2 soon.