The company was setup like a collection of startups. You would pitch your ideas to executive management who would green light hiring and what not. You'd also borrow or entice engineers over to your group. When I joined, it sounded very interesting, but it was a complete nightmare.
The executive management would green light things, but they'd green light 10x more than could be done by the engineering staff. The hiring queue was so long that you'd be lucky to see engineers hired for your project for 6 months. That meant stealing engineers from other groups which encouraged serious protectionism by doing things like say, loading down your group with busy work to keep your empire intact.
It was a political nightmare because no one was really in charge, so everyone claimed ownership. You had review processes for every single step of development by whatever group claimed ownership over something you touched regardless of how trivial.
The executive management rarely stuck their necks out making decisions. You had to prove something was better with data. The fact that data was easily manipulated didn't bother anyone, as long as they didn't have to take any blame.
The thing is, the company itself is wildly successful and could be held up as a model for flat organizations if you didn't look too closely. But really, it is in spite of their organization rather than because of it.