This is different from the Wayland security model, as Wayland restricts the ability for clients to modify and read from other clients arbitrarily. This is an extension to a Wayland compositor, and as all extensions do, it contains code which runs on your system. Any code, unless sandboxed, can access your filesystem no matter if it's run under Wayland, X11, or no windowing system at all for that matter.
wayland was not about avoiding applications accessing resources or running scripts (that is why sandboxing is for) but to avoid programs to have access to the rest of the graphical session (things like input devices and other graphical windows and their data)
It does, when the bad actor is a program you run, and other open windows contain sensitive content.
Here the bad actor is code being loaded as an extension to the compositor. A bit like a kernel module, which can bypass file access permissions if it wants.