The documentation for the $CFG->lock_factory config setting in config-dist.php is:

// Moodle 2.7 introduces a locking api for critical tasks (e.g. cron).
// The default locking system to use is DB locking for MySQL and Postgres, and File
// locking for Oracle and SQLServer. If $CFG->preventfilelocking is set, then the default
// will always be DB locking.

This is not what actually happens in lock_config::get_lock_factory(), as only Postgres has its own specific implementation. It should be something like:

// Moodle 2.7 introduces a locking api for critical tasks (e.g. cron).
// The default locking system to use is DB locking for Postgres, and file locking for
// MySQL, Oracle and SQLServer. If $CFG->preventfilelocking is set, then the default
// will always be DB locking.