The main difference, and what got me to start this project, is that you can layer on packages. This means no more dealing with disabling read-only and losing your changes every time you update.
Besides that, this incorporates a huge number of community made features such as SDGyroDSU and Discover Overlay OOTB, a newer kernel than even SteamOS 3.6, DisplayLink support, Nvidia support (on desktop), and so on.
It’s essentially become an immutable/atomic gaming spin on Fedora, with full support for the deck.