There is no such rumour since Apple has simply announced this feature at last years WWDC where they have explained what it is and which of their own apps are using Marzipan (the ones you mention here).
The only thing that can be seen as a rumour is that this years WWDC will see iOS 13 and macOS 10.15 with this new feature. I must say that this is more an educated guess than a rumour though.
That’s a simplification of what developers need to do. What Apple has pointed out last year is that it is rather easy to create these Marzipan apps. However, that might be different for the developer because they may have to break down their apps (modularise them) which may not be very easy to do (especially those with monolithic apps).
What Marzipan means is rather different from what you think. Marzipan is meant to make things easier for developers. What developers currently have to do is develop a new app for iOS and macOS from the ground up. What Marzipan provides them is that they no longer have to do that when they already have an app on one of the two platforms. You can share certain parts of the app (libraries and such). As Apple has stated, apps for iPhone/iPad have to use a completely different way of interfacing than apps for macOS. This is not going to change. Developers still have to design an interface that works on iOS and one that works on macOS.
It is simply standardising and unifying the foundation of iOS and macOS apps. Currently those are completely different beasts. Car manufacturers have been doing this for some years now (using the same chassis/engine throughout various models of cars) and have stepped it up with electric cars where they are designing an undercarriage where you can put on whatever module you want. By doing so you can skip the development of the undercarriage with each new car you want to design. You only need to focus on the bodywork since the undercarriage is a given. That is exactly what Marzipan is.
This has got nothing to do with it being Marzipan or not. It is simply a matter of not having implemented such functionality. The latter is what is discussed here in this topic thus something like Marzipan is actually offtopic.
Edit: apparently so much offtopic that the Marzipan discussion has now been given its own topic. So I guess…fire away?
