Except Marzipan is about UIKit on the Mac so it will definitely be easier to port an iPad app to the Mac that it would be to port a Mac Appkit app to the iPad.
That said, a developer now has fewer hurdles in developing an iOS version of their app because if they rewrite it in Marzipan, they can continue to develop their macOS and iOS apps in parallel, and they can just drop support for the pre-Marzipan macOS version of the app.
