I believe the author to be misinformed about manufacturing a firearm.  As long as you do not sell it I believe it to be legal. There was a company from Montanta that sold the blanks to manufacture all sorts of weapons legally.

This is true. I have heard on good authority that it is completely legal to manufacture your own firearms (as long as they are not machine guns) as long as you don't sell them. There are companies out there that sell "80% finished receivers" for a variety of guns, including the AR-15. It is not completely finished, so they aren't selling a receiver. You just finish it up and add the rest of the parts. So I don't think making one of these using this method would be illegal either.