Probably not a mistake. The Gauss Rifle has always been TL-12.
And generally replaces ACRs on TL-13:
LBB4, p43 wrote: Tech level 12:The gauss rifle is introduced in limited numbers as a sniper weapon, expense precluding general issue. The individual grav belt is occasionally used for scouting purposes.
Tech level 13: AII infantry is generally now in combat armor and equipped with gauss rifles. Battle dress is issued to selected assault troops.