As mentioned to open, containers within VMs are a security standard for cloud-native when security is critical.

x11docker is just a (very convenient) security layer for containers which need to expose graphics (and possibly webcam, audio, networking, clipboard, printers...). Kata Containers are just "micro VMs" where you spin up a separate kernel to drop the container into.

Bubblewrap is okay if you trust your kernel, but locking the app away in its own VM with its own kernel gives another layer to bust through.