You are correct. Settings > Printers > Machine Settings > Extruder.
I’ve also seen something on the Forum that indicates that Luban will get support for 0.6mm nozzles. IIRC, it was a drop down, only allowing 0.4mm and 0.6mm. I don’t remember where I saw it though.
Thanks for the link @Hauke !
Edit to add: The Artisan product page mentions 0.2, 0.4, 0.6, and 0.8mm nozzles. So I expect Luban will support them all eventually. With the dual extruder supports in Artisan, it might even support them at the same time.