Not sure what the poster meant but DuckDB is an analytics DB, it doesn’t have a btree index - at least not last time I looked. You could consider it the OLAP embedded DB to SQLite’s OLTP embedded db.
DuckDB can read SQLite so you can even imagine using them side by side in the same system, serving point reads and writes from SQLite and using DuckDB for stuff like aggregates and searches that SQLite is slower at.
They are complementary to each other. There's an SQLite extension for use within DuckDB [1], which gives you a power of great transactional capabilities of SQLite and speed of analytical queries within DuckDB's columnar storage engine, all within a single database.
