It certainly doesn't have to be the case. The MongoDB layer can be used via the MongoDB protocol. TiDB is an entire MySQL implementation that any MySQL client can connect to which is built on top of TiKV (equivalent to FDB). If the goal is lower overhead of data transfer, another approach is writing layers in C (or Rust exposed as C) and then generating bindings for different languages.
Comments
It certainly doesn't have to be the case. The MongoDB layer can be used via the MongoDB protocol. TiDB is an entire MySQL implementation that any MySQL client can connect to which is built on top of TiKV (equivalent to FDB). If the goal is lower overhead of data transfer, another approach is writing layers in C (or Rust exposed as C) and then generating bindings for different languages.