Yeah, but it's not just about solving the problem "how do I install package X on a random Linux distribution". I agree there have been solutions for that for a long time. In fact, Snap at least does not even fully solve this problem because its sandboxing depends on AppArmor, which clashes with SELinux on distros that use this - (e.g. anything from Red Hat).

The real key to Flatpak and Snap is the entire app "experience" - integration in the software center as a trusted source, automatic updates, clean installs and removals without messing up other parts of the system. Before Flatpak and Snap, this did not exist on Linux, especially not in something that was compatible with all distros.