What you're talking about works OK in a single colo or datacenter model, but it doesn't map to the PoP model. It's the PoP model that's driving them to do this. In their PoPs they've got local peers and exchanges that will handle the bulk of their traffic. The leftover stuff is only local and probably minor. They can definitely find that with flow data and optimize across a pair of default routes (basically building their own routing table). It's a pretty common practice when all you deploy is PoPs. Why do you care about a full BGP feed when you're only handling 4-10% of the Internet in a given PoP?
AS path length doesn't tell you anything about a route's performance. It's a hint and only a hint.
> What you're talking about works OK in a single colo or datacenter model, but it doesn't map to the PoP model. It's the PoP model that's driving them to do this
Could you please explain how the PoP model in this case differs from the colo/DC model?
Outside the US Fastly's PoP cover multiple countries or whole continents.
AS path length doesn't tell you anything about a route's performance. It's a hint and only a hint.