Under the "App Icons and Launch Images" section of the General project settings there is just 1 drop down menu to select the App Icons Source. How can I specify both iOS and macOS icon sources for a Catalyst app?
How do I specify a Mac App Icon for a Catalyst App?
I discovered the solution for this. Simply open the attributes panel on the AppIcon in the assets browser and check the Mac checkbox to add the Mac specific icon placeholders.
Talk about obscure. I especially appreciate having my time wasted by the Xcode "validation" process's reporting success and uploading the whole product, only to be told on submission that an obvious item is missing.