It's specifically the EP Arms 80% poly lower not the aluminum lowers. They claim EP is manufacturing the lower as mostly complete, including fire control pocket and pre-drilling the pin holes, and then filling the cavity with a different color plastic. According to what EP has published, this is a false statement and they actually build the "plug" for the pocket and then form the lower around it. Having built one myself, I can tell you there is no way to mill this lower without doing everything from milling the pocket and drilling the pins. There is no way to get the plug out, and it is fused to the lower with cross bars through the plug from the lower injection process.
But it doesn't matter at this point. With everything that has happened I don't see the ATF giving up on this and they WILL overturn their previous declaration on this type of 80% lower, saying they are firearms and must be registered. It won't just be EP, it will be all of the others as well. Other manufacturers and dealers have also pulled their 80% lowers in a response to this.