Yes I have. I think they do the same as OpenSUSE microOS basically.
The live system is immutable, when updating they clone it to the other partition and run regular apt in there. (Not sure about that but I think). Same issue as on OpenSUSE [whatever they want to call these variants].
It sounds like the thing Android is doing, but in detail it is way less secure. I only know of rollback prevention and signing, so an update needs to be an update and cannot downgrade components. This may not be available there but idk.
And the entire boot process on any Linux distro is extremely insecure compared to Android/GrapheneOS on a Pixel.
Their "apk package manager" is just a wrapper for Distrobox, not solving any fundamental problem. But Distrobox for sure is awesome for closing the gaps.
I think uBlue with homebrew is doing something more sustainable here though, as homebrew is independent, well maintained (cross OS!) and does not rely on having a separate OS run in parallel. So if you imagine Fedora would only supply base packages in some future, a project like homebrew could take care of the rest.
Also I couldnt even get the Debian version installed in a Qemu VM, same as with EndlessOS, so yeah so much about "alternative immutable distros".