From the photos it appears that dude was using a harbor freight drill press with a cross slide vise and end mill to "machine" the receiver cavities. That's a sketchy looking setup.

here's how the ATF determines whether a lower reciever is classified as a firearm or not:

https://www.atf.gov/firearms/qa/are-%E2%80%9C80%E2%80%9D-or-%E2%80%9Cunfinished%E2%80%9D-receivers-illegal