You are mixing apples and oranges.

From the page you quoted from, the title of that section is..:
Q: Is it legal to assemble a firearm from commercially available parts kits that can be purchased via internet or shotgun news?

That section pertains to ASSEMBLING a firearm from pre-manufactured parts. 3D printing is actually manufacturing the parts by any definition I can think of. Different scenario than what the article refers to.

And, by chance did you read the VERY FIRST sentence in the article you quoted from? Here it is:
For your information, per provisions of the Gun Control Act (GCA) of 1968, 18 U.S.C. Chapter 44, an unlicensed individual may make a “firearm” as defined in the GCA for his own personal use, but not for sale or distribution.
By the way, the ATF bolded those words. Not me.

That statement verifies what I have said all along. You can make a gun. You can't then sell or give it away unless its properly marked and serialized.