No Lesser Than John Gruber of the Daring Fireball believes that cross-platform apps for iOS and macOS are only over in the past Next version of iOS would appear and the feature "sounds like a declarative control API". At least that's what he learned from his sources.
Such a control API would make sense, for example, by addressing the API differences between iOS and macOS, or perhaps enabling a cross-platform UI framework. The latter should be the biggest hurdle - because macOS is not made for finger control and iOS is not for keyboard control.
Gruber then goes on to say that he is "almost sure" that this feature will not be demonstrated at WWDC this year, but will be in 2019 with macOS 10.15 and iOS 13. The first reports predicted a start in 2018.
The report draws a different picture from Bloomberg's first report describing Apple's marzipan project as one that supports cross-platform applications running on iPhone, iPad, and Mac. In these reports, it is important to note how the same information can be interpreted from different sources and how things change as these features evolve.
Whether at WWDC in June, we will see cross-platform features of all kinds remains to be seen.