YouTube is connected to every transit ISP out there and can send through different routes at will. Are the ISPs using some sort of shaper that only impacts YouTube?
Pretty much. Shaping the CDN is good enough to get the job done. Least they're only shaping... not that such is good. We've had instances at work where our entire CDN is blocked by certain ISPs in certain countries who will go unnamed (this makes it HELL to support our international users).