That’s true, but some system level changes by default can’t be done because most system folders are read only. It’s trivial to turn this off, however a steam os update would overwrite any changes made.
For installing new apps this is not an issue because Steam os is designed with flatpak support, so if an app is shipped as flatpak (or appimage which is just a single file you can execute) then it can be used without making the system folders writeable
However in this case, it’s about the nix package manager which needs access to specific system folders to not just install packages (which can be apps or system stuff) but also to apply changes to the system configuration
It’s really nice that a valve developer is taking the extra steps to make sure nix can be used out of the box. I don’t really understand why they are doing this, I think it’s just because they think it’s cool and some users are going to appreciate it.
Check out distrobox. It’s a way to have a Ubuntu (or any other Linux distro) container and allows you to install Ubuntu packages, even desktop applications.
It works great for when you need to install a random .deb file or follow a very Ubuntu specific step by step procedure. I use it exactly for this kind of stuff.
No rebooting needed, integrates fully with the host system, no virtual machine either.