It's backed by the JSON extension. You can use autoids to ensure all data is insert only. I'm sure this project has some really nice features but for the schemaless piece, just saying that SQLite has a similar component in case some don't know.
That said, the significant dialect difference is that Endb's JSON-like documents are strongly-typed. One of the places the Endb SQL dialect diverges from SQLite's is typing. In this respect, SQLite is "more schemaless"... at least at the level of weakly-typed scalars and nested JSON (since JSON supports a diminutive set of types).
Endb's data types, their exposed JSON-LD types, and internals: https://docs.endatabas.com/reference/data_types.html
Endb's SQL data types: https://docs.endatabas.com/sql/data_types.html
...all of these can be used at any nesting level, which makes nested data in Endb quite different from JSON.
Hope that helps explain the difference!
(I'm involved in the Endatabas project and happy to answer other questions or clarify this.)