Arch was likely chosen because arch-based SteamOS has already pulled it off (immutable system including KDE).
Lets them waste less effort reinventing the wheel.
Is Aurora more stable than Fedora Kinoite?
The Aurora image includes some necessities. The Aurora NVIDIA image includes drivers and should just work. You can theoretically also rebase to another version, such as Bazzite, with a different desktop environment.
Considering they're aiming for reliability I would've liked if they'd built on a truly reliable atomic distro base.
https://getaurora.dev has been so damn good, but despite being a Plasma distro it still packs a lot of GTK apps from Bazzite for basic functionality.