I am lucky enough to have worked on all three of these systems (TAO, ZippyDB, and currently MySQL) so can shed some light here.
Both MySQL and ZippyDB are datastores that use RocksDB under the hood, in a slightly different way and with different querying capabilities exposed to the end user. ZippyDB uses it exclusively, but MySQL uses both the traditional InnoDB and RocksDB (MyRocks). TAO is in memory graph database, layer above both of these, and doesn't persist anything by itself - it talks to the database layer (MyRocks).
Both MySQL and ZippyDB are datastores that use RocksDB under the hood, in a slightly different way and with different querying capabilities exposed to the end user. ZippyDB uses it exclusively, but MySQL uses both the traditional InnoDB and RocksDB (MyRocks). TAO is in memory graph database, layer above both of these, and doesn't persist anything by itself - it talks to the database layer (MyRocks).