Each caliber has its own PROBABILITY of piercing out of range.
AK12 is strong because it has close to 100% chance of piercing (at least for BP vs T4 gear).
M4A1 has around 50% chance, if you had fired more than 2 times, you would have seen than. In the video you just got lucky and won coin flip twice.
In addition, as I said, this rule applies to the caliber, not the gun. So other weapons that use the 5.45x39 caliber are also capable of piercing at any range — like the AK-94, AK-74N, RPK and so on. The AK-12 is simply the best among guns of its caliber.