GaleraMon's documentation on available_when_donor says:

available_when_donor

This option only has an effect if there is a single Galera node being backed up an XtraBackup instance. This causes the initial node to go into Donor state which would normally prevent if from being marked as a valid server inside MaxScale. If this option is enabled, a single node in Donor state where the method is XtraBackup will be kept in Synced state.

available_when_donor=true

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-galera-monitor/#available_when_donor

It specifically mentions that the SST method should be XtraBackup. It should also be updated to mention that it can apply when the SST method is mariabackup.

I think it should also specifically mention for less knowledgeable users that wsrep_sst_method is the variable that they need to check on the backend nodes.

https://mariadb.com/kb/en/library/galera-cluster-system-variables/#wsrep_sst_method