I doubt they are querying 1 TB of data in the browser. DuckDB-WASM issues http range requests on behalf of client to request only the bytes required, especially handy with parquet files (columnar format) that will exclude columns you don't even need.
But the article is a little light on technical details. In some cases it might make sense to bring the entire file client-side.
For small databases, SQLite is handy, as there are multiple ways to parse the format for clients.