I disagree. CSVs don't have write-ahead logging and are not able to selectively lock portions file for writes.
And I'd say that not only is sqlite in the db marketplace (albeit for a specific subset of database application types) it's one of the largest players.
Why not? Just roll your own write-ahead log when you are writing to CSVs! That's all the SQLite code is doing.
SQLite does not selectively lock portions of the file for writes. It locks the entire file using the Operating System's own file handling services.
SQLite is a file format. CSVs, XML and JSON are also huge in the dB marketplace. That doesn't make them RDBMSs.
A collection of CSVs can be used as a database.
SQLite is not in the DB marketplace. It's in the file format marketplace. It handles multiple writes in exactly the same way CSV handles multiple writes. If you want to handle multiple writes with SQLite, you handle it the same way as CSV.