What I want to do is define these three settings by defining a new array for the configuration parameter:
$wgHostStatsCommands = [ 'df -H', 'lsb_release -d', 'date' ];
What happens is that also the default settings are considered, i.e. in the end 'hostname', 'df-h', 'df -H', 'lsb_release -d', 'date' are shown instead of just 'df -H', 'lsb_release -d', 'date' as desired. This started after the conversion to the extension registration system. Apparently I have no clue how to fix the extension.json file, i.e. how to disable any existing merge strategy to "config".