You mean that you can load a SMuFL font in Dorico even if this font does not have a json file for Metadata?
I cannot do this on my system and I am quite sure that a SMuFL font does need a Metadata json file to be recognized by Dorico.
I will check later.
@Vesa
In case you have problem to create a json metadata file, you can have a look to this sthread:
You will find there some instructions on how to create the most minimal json file