On X11, what I described can be done in a stable, standardized way; on Wayland, it requires hacking the compositor's internals in semi-supported, unstable ways.
These are the differences in features whereof I spoke that make Wayland and unrealistic platform for many users in it's current state.
The “if” on X11 at this stage is a theoretical obscurity with every modern window manager of note supporting it, with the user being confident that it shall remain stable; on Wayland, the reality is that every single compositor has it's own own, typically unstable ways of doing this, though Sway is the one that is committed to stabilizing it and wants it's way of doing these things to become a standard, but that is of little use when the others are not picking it up.
I would not say that Sway is doing anything special here, what they have done is exactly what I was saying earlier: placed the unstable private APIs in the Wayland protocol, with all the problems that entails. Applications should probably not be consuming those APIs directly, they will likely want to go through an abstraction layer.