In the context of this thread (and many other on the forums here) cross-platform tends to mean OS library backend/vendor (Generally meaning Apple, Linux, and Microsoft).
Apple-platforms may vary drastically but they are all backed by the
Darwin C library and so they really have a more lot in common when it comes to programming for them. It is frequently sufficient to compile frameworks using something like the below code snippet and have it work on all (or at least most) Apple and Linux platforms:
This definitely indicates that the true difference lies not necessarily in the OS, but in the backend OS C libraries (sometimes it does, but not always). Looking at several cross-platform swift libraries makes it apparent that most compiler flags are used to distinguish between apple and linux platforms and far less frequently between the individual apple platforms.
Each Apple platform may vary drastically but they all use flavors of
Darwin as the backing C library while Linux uses
Glibc and Windows uses it's own thing (which I can't remember off the top of my head right now). These backend C libraries which are used by swift are what really determine the platform behaviors/capabilities.
No where have people said that it means they should share 100% of the code, but rather 100% of the same functionality and behavior. No one thinks that it would be possible for them to share 100% of the same code. Look at any cross-platform swift framework and see all the
#ifs that it takes to get things working on both Apple and Linux. It has even been stated by multiple proponents of making SwiftUI open source and cross platform that it would/should use the platform's UI framework, but would only enable the same functionality for building the UI.
Yes SwiftUI is "cross-platform" in the sense that it supports all of Apple's various platforms, but it is not in the sense that it doesn't support any platform C library except for Apple's
Darwin. This is what people are arguing for and have stated multiple times for various reasons.
You can easily write a framework in pure swift that supports all Darwin-based devices and technically call it "cross-platform". Getting a framework that works on more than just Darwin devices usually takes a bit more work and customization due to the variations in C backends, availability/stability of Foundation, etc.
What I understand that people are really asking for is a common framework for building UIs and not a single UI framework as that would be impossible due to all the variations in how OS's generate graphics.