Sounds like they provide a caching layer in between

"Every blob uploaded is stored in-memory to provide fast access — and then blobs that are not read frequently are eventually automatically moved to disk for low-cost, long-term storage. When you make a new read request to a blob in disk, it is reloaded back to memory, providing you the fastest possible access again."

For a developer-focused tool, it strikes me as odd that they wouldn't just use the term "caching" though?