I think that’s only true for the programs, not for the JVM/JRE code. The JVM/JRE doesn’t support Wayland without the xwayland compatibility layer. Also, some games use “native” libs that do optimized 3D stuff. Those are special Java classes, not part of the JVM/JRE that interface with C libs, kernels, system calls and hardware directly. Some will stop working without an X window to connect to. Some are long forgotten and won’t be ported.