This is, IMO, the biggest yet least obvious advantage of immutable systems. A traditional Linux environment is “just as safe” as the immutable setups, if only the user/administrator is perfect, never makes a mistake, and always makes the right decisions for now and the future.
Given reality tends to differ from the above, having a system that, at a bare minimum, provides you the “oh shit go back” button to system-level changes, and at best provides a clear, reproducible, trail of actions, is a huge advantage for long-term stability for all users, experienced or not. I’ve been through the school of hard knocks far too many times maintaining everything from server setups to gaming desktops the traditional way, and have committed to “early adopting” immutable distros for pretty much everything except the gaming setup (given the whole suite of proprietary and out-of-date/out-of-touch applications that are basically necessary in that space and not-fully-compatible with the sandboxes and abstraction layers necessary).
In this case it’s referring to the fact that the OS is built upon the same containerization technology used on cloud platforms such as Kubernetes. As a marketing tool it’s a bit buzzwordy, but it’s not about running the core OS components outside of the physical machine here.