It seems like a growing number of things once referred to as Linux distributions are now referring to themselves as operating systems. If the kernel is Linux, and the user-space is GNU, what makes this a distinct operating system from, say, SUSE, or Arch?
Some distributions are operating systems (eg: OpenBSD, ArchLinux, Debian). Some operating systems are not distributions (they don’t include a mechanism to pull packages. Eg: windows, macOS). Some distributions are not operating systems (eg: homebrew, Flatpak).
Tails focuses on the operating system side of things. It’s focus isn’t on package distribution and letting you install things, but on downloading a usable OS image. It’s still a distribution, but that’s more of a technicality.