This scale is only possible because of what they don't ask Postgres to do.

If you treat Postgres strictly as a Key-Value store for transactional integrity, it flies. But if you try to run complex discovery or tsvector search on that same primary, the locking/CPU load kills it instantly.

The real hero here is aggressive segregation: keep Postgres for 'Truth', and offload all discovery to specialized indices that don't block the write-head.