The "mistake" referred to is that the 2014 published regulations were issued without the references to exemptions of certain "armor-piercing" ammunition, leading some to conclude that these had been rescinded, which is not the case. The "green tip" 5.56 mm ammo is, in fact, currently exempted from the ban. Note the wording from this publication:
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Please be advised that ATF has not rescinded any Armor Piercing Ammunition exemption, and the fact they are not listed in the 2014 online edition of the regulations, was an error, which has no legal impact on the validity of the exemptions. The existing exemptions for armor piercing ammunition, which apply to 5.56 mm (.223) SS 109 and M855 projectiles (identified by a green coating on the projectile tip), and the U.S .30-06 M2AP projectile (identified by a black coating on the projectile tip), remain in effect.
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In other words, there currently exists an administrative exemption for a few specific ammunition types that are otherwise banned because of the "legitimate sporting purposes" standard. Thus the current debate revolves around cancelling that specific exemption rather than imposing a new ban.
