That's not how I think of it. By analogy, consider POSIX. Every major operating system supports the POSIX interface--all of them, except one that is. But no one goes around calling that one OS a "giant, existential, ecosystem-fragmenting threat to the open-source community."

Essentially, you have to pick a point somewhere in the stack where you define the common interface between the different hardware vendors. The problem is that one hardware vendor, because it's the 800 lb. gorilla who loves throwing its weight around, decided to not implement the common interface, hoping that its weight will force everyone else to adapt. Wayland is unfortunate enough to be in a position where it relies on that common interface being supported, but this isn't the only place that NVidia has decided to try to scuttle cross-vendor compatibility (GPGPU computing is a very notorious case).