Flatpak still depends on runtimes though, I have a few different runtimes I had to install just because of one or two flatpaks that required them (like for example I have both the gnome and kde flatpak runtimes, despite not running either of those desktop environments)... and they can depend on specific versions of runtimes too! I remember one time flatpak recommended me to uninstall one flatpak program I had because it depended on a deprecated runtime that was no longer supported.
Also, some flatpaks can depend on another flatpak, like how for Godot they are preparing a "parent" flatpak (I don't remember the terminology) that godot games can depend on in order to reduce redundancies when having multiple godot games installed.
Because of those things, you are still likely to require a flatpak remote configured and an internet connection when you install a flatpak. It's not really a fully self contained thing.
Appimages are more self contained.. but even those might make assumptions on what libraries the system might have, which makes them not as universal as they might seem. That or the file needs to be really big, unnecessarily so. Usually, a combination or compromise between both problems, at the discretion of the dev doing the packaging.
The advantage with Nix is that it's more efficient with the users space (because it makes sure you don't get the exact same version of a library installed twice), while making it impossible to have a dependency conflict regardless of how old or new is what you wanna install (which is something the package manager from your typical distro can't do).