I'm not extremely experienced in building them, but I've done a TON of reading. I've learned to use many sources and filter out the stuff that doesn't seem right when researching. You can certainly filter my info if you like.
While I say he's wrong about only 2-3 makers of uppers and lowers, he's kind of right about them not being extremely important of the maker of the upper and lower, if you're talking stripped receivers. Lower is less important than the upper. The important things are the items that go in the receivers. Aero Precision makes a lot of the receivers that get roll marked for various companies and sold by them as their brand. He's probably right in the sense that a few companies produce the large majority of stripped uppers and lowers on the market. Arguably the two most important components of an AR are the bolt carrier group and barrel. BCM BCGs are highly regarded. I've heard good things about the Daniel Defense BCGs, Colt BCGs as well as the PSA BCGs. Some say the PSA BCGs are made by DD but PSA won't confirm and I understand the reason. I have a factory BCM rifle and am very pleased with the BCG in it. I have a BCM BCG I'm using in a build right now.
I would question whether a small scale builder could assemble an AR for $650 with decent quality parts and turn a profit. I'd be pressed to assemble one for $700 and that would be with catching sales.