The distinction between iPhone and iPad support sticks out. In the age of Split View, iPad app have to be ready to adapt to skinny columns, square, and full-screen layouts at any time. They are programmed to be continuously resizable. Even fixed size iPhone apps on macOS would be alright; they’d just be small windows amongst your other windows.
In an attempt to decipher the Chinese whispers here, I think what this actually means is that this year’s Marzipan system will not let you shrink windows into single-column designs. At a technical level, apps running under Marzipan on macOS 10.15 will not transition to compact width size classes. In fact, this is exactly how News, Stocks, Voice Memos and Home work on Mojave. Try it right now. Although the windows are resizable, they have minimum size constraints in both width and height. It is not possible to compress the interface enough to collapse the sidebars.
I don’t know why they chose to enforce this restriction as many actual Mac apps, like Finder, do happily morph into skinny column layouts when their windows are made small, and I can’t think up a technical reason that would prevent it from being implemented.