No, I think he's right. I did a fair bit of reading up on this before I started milling out my 80% lowers, and I've paid attention since because I've got a couple more on the shelf for future builds.
An 80% lower is a paperweight. Once you start machining it to turn it into a receiver, you're the "manufacturer". It's perfectly legal for you to "manufacture" a firearm for your own use. And in doing so, it doesn't have to be serialized. You can't sell that unserialized lower to anyone else. That's federal law as defined by the ATF.
Most states go along with this. Massachusetts is a little weird, they don't consider the receiver the "firearm". And apparently you have to EFA10 the firearm within 7 days after completing it. (??). I'm definitely not an expert on this. I don't live in Massachusetts.
That being said. If you manufacture, or help manufacture a firearm for someone else - you're now crossing lines with the ATF - you're now a MANUFACTURER. That's a problem. It doesn't matter if you're a licensed gunsmith or an amateur, you're still a "manufacturer" - and you have to be licensed as such.
That's what thrown such a crimp into "build parties". What was happening in some places was that machine shops were getting together a group of guys who didn't know anything, and having them basically "touch the 80%" and saying that they did the work themselves. The ATF got wind of this and got pissed off.
There's no welding involved in doing an 80%; it's material removal. If you trash an 80% lower, it's just a chunk of aluminum, it's not a firearm, so you go get somebody who can weld aluminum to flow a blob back onto it and start over. Or buy another one, they're not horrendously expensive.