Can you point me to what gave you this impression? We implement our own query engine for Triplit and use a fork of tuple-database[1] for storage which is low level database that can store to SQLite (we do on the server) but in the browser it uses either an in-memory btree or IndexedDB.
Regarding compatibility with our databases, we do have some internal experiments of running a Triplit Server in front of existing databases as almost like a syncing cache. Still very experimental but maybe you'd be more into that use case.