Why that's a surprise to anyone, I don't know. Very very few companies make every component of their product, because doing so is horribly inefficient. Bushmaster sells around 70,000 AR lowers a year, but it's STILL more efficient to out-source the lower machining to a company that does nothing but machine work, and can therefore afford the ultra-high-end computer-controlled mills that can make each lower exactly like the one before it.
What *matters* is the QA checks along the way. Does it matter if the lower was machined by Continental or Lewis, or filed on by hand by an old man in Afghanistan, as long as it meets specs exactly? Not really.
As long as a company is ensuring those parts are held to the required tolerances and specs, then where it was machined is irrelevant.
Oly is known for machining much of their products in-house, yet back when they machined their own lowers, they were well-known for making out-of-spec lowers. That's because a $10,000,000 CNC machine will give you much better consistancy than a $50,000 Bridgeport and a guy. That's why we have the computers do it.
-Troy