Comment by pmontra

If by solving multinenancy they mean

  CREATE TABLE tenants (
    id     BIGINT NOT NULL,
    database SQLITE DEFAULT execute_sqlite(
        empty_sqlite(),
        'CREATE TABLE users (etc.)'
        and all the other tables
        for each tenant
    )
  );

then they don't need to make joins between sqlite dbs.

Your other concerns are very real. Those sqlite dbs could become very large. I prefer the use case depicted in another reply: preparing sqlite dbs before shipping them to their own devices. Or maybe receiving them and performing analysis, maybe after having imported it in overall psql tables. Or similar scenarios in which all the db is read or written at once. Anyway, once we have a tool we start using it.