In the meantime you can change the diameter manually in the settings:
Open snapmaker_extruder_0.def.json under
C:\Program Files\Snapmaker Luban\resources\app\resources\CuraEngine\Config (or where ever you installed it)
change the parameter “machine_nozzle_size”:
"machine_nozzle_size": { "default_value": 0.4 },
Then restart Luban and it should take the settings.