I don't really see the logics behind this example, tbh. Umbrel is merely a wrapper that installs apps in containers through scripts, it highlight convenience and ease of use (especially compared to having to setup docker networking on your own for bitcoin apps). This is more similar to the behavior of any other desktop distro, which allow users to install apps through a store GUI. NixOS is radically different. The strength is indeed in the config file, which allows to replicate a system no matter the original configuration. For this, its main target is developers and IT professionals, but not average joe (unlike Umbrel).