The documentation for the audit plugin says the following about server_audit_incl_users / server_audit_excl_users:

Logging is also based on username only, not the username and hostname combination that MariaDB uses to determine privileges.

https://mariadb.com/kb/en/mariadb/about-the-mariadb-audit-plugin/#excluding-or-including-users

In addition to including/excluding certain user accounts from auditing, some audit plugin users would also like to include or exclude certain host names from auditing.