I don't envy their position of having to scale that fast on something that has to be instant and real-time. As far as I know, you can't do CDN/edge caching shenanigans with a remote git repository like Google can with a YouTube video. It's gotta always be reading/writing to the latest, single source of truth.
The easy solutions like caching and read replicas don't work and you're forced to go the route of sharding or similar techniques that have much more painful tradeoffs.
I'm not sure if that's why everything keeps breaking but at that scale write-heavy workloads are never going to be easy