Sorry but you're wrong, Flatpak has been around for longer than that, specifically at least 2014 and was known as xdg-app before https://github.com/alexlarsson/xdg-app/commit/a640cd365bd217...

And if you look at the history page of Flatpak, you'll see that the project has been in development in some form or another for roughly 20 years https://github.com/flatpak/flatpak/wiki/Flatpak's-History