Percona has their own backup locks implementation. Percona's xtrabackup is supposed to be worked not only with Percona Server, but also with MySQL and MariaDB. So the code detects if server supports Percona's backup locks, then it uses them instead of FTWRL.

There is no need in that code in mariabackup, as it does not support Percona's backup locks.

There is also --no-backup-locks option. The option disables such detection, and it's supposed that FTWRL will be used instead.

See lock_tables() function and the code around have_backup_locks global variable for details.

This is dead code for mariabackup, so it can be deleted along with --no-backup-locks option.