I am not sure that is fully true. Or at least not fully explained. The Steamdeck has a full KDE environment installed and it uses this when in desktop mode. But steam is not running in big picture mode in front of this.
KDE is not running at all when in the game mode of the Steamdeck. In that mode it uses a compositor written by valve called gamescope. Switching between these is effectively logging out and back in again to switch the compositor.
Also it now has a way to run the desktop as a nested session in game mode but that is winning kwin inside gamescope.


LSF is not a distro. It is a instruction manual and teaching aid. Don’t use it as a base for you main OS. And IMO Gentoo does not really teach you more then Arch does. It gives a bit of flexibility that not many care about (how things are compiled) at a very big cost (of having to compile everything yourself). I would not use either unless compiling things is your hobby.
Sure, try them in a VM if you really want to. But I would not really consider that moving on from your current distro nor do you really need to do that.