Author here! We were really motivated to turn a "distributed system" problem into a "monolithic system" from an operations perspective and felt this was achievable with current hardware, which is why we went with in-process, embedded storage systems like RocksDB and Tantivy.

Memory-mapping lets us get pretty far, even with global coverage. We are always able to add more RAM, especially since we're running in the cloud.

Backfills and data updates are also trivial and can be performed in an "immutable" way without having to reason about what's currently in ES/Mongo, we just re-index everything with the same binary in a separate node and ship the final assets to S3.