I think you're misunderstanding the meaning of "Software freedom". It refers more to the process and transparency of the software than to the choice. That said, the Steam Deck is just a handheld PC. And Valve gives instructions on how to install Windows on it should you want to do that. https://help.steampowered.com/en/faqs/view/6121-ECCD-D643-BA...

Often we talk about software freedom in the context of open-source development and free-software licenses like the GPL. The Free Software Foundation stated as a bootstrapping organization to write open source software for the GNU platform (Linux/Unix standard userspace environment). https://en.wikipedia.org/wiki/Free_Software_Foundation

Valve is pretty well respected from that perspective. SteamOS is built on Arch Linux. They publish the source for most of their Linux tools https://github.com/ValveSoftware/. The development of Proton, their in-house compatibility layer that uses Wine under the hood, is also open source and developed with community involvement. Single hardware platform makes it easier to handle the morass of driver development. They upstream their changes to other projects. There are actually open source forks of things like Proton (https://github.com/GloriousEggroll/proton-ge-custom is a popular one).