There's still a lot of ways the ATF can show up and shoot your dog if you pick the wrong components and put them together without paying $200 and waiting half an eternity.
The question, though, wasn’t whether ATF agents might interpret the regulations one way or another, but what the regulations are. Millions of people purchase legal and freely available components for their legal firearm with a serial number, and do not run afoul of the law. As soon as we are talking about components or firearms requiring a NFA tax stamp we’re in a different category though.