Good question. Flatpak doesn’t lead to fewer updates overall, but it does lead to fewer system packages installed via pacman or yay, which can run into dependency conflicts unlike Flatpak.
Flatpak provides a common runtime upon which different applications can be installed via containers, much like apps on a phone. You can then adjust the permissions for each app such as which directories it can access. It’s kind of like installing Firefox (e.g. Flatpak) and then a Firefox extension such as uBlock Origin (e.g. LibreOffice). It doesn’t matter if you’re on Ubuntu, Fedora, or Arch, from the extension’s perspective it’s the same old Firefox and doesn’t touch anything else on your computer. This means uninstalling is clean and it has no dependencies on other packages installed.
The disadvantage with Flatpak is they can be slower (sometimes not even noticeable) and take up more space, although the runtime can be re-used between Flatpak applications. Personally, I like Flatpak for large GUI applications like LibreOffice, which has 170+ dependencies if installed via pacman.