To be clear, this is just an alternative Linux OS. The hardware shown in the thumbnail and article is just a mockup meant to represent a potential handheld. Part of their goal is to offer equally good support for all PC game stores (epic etc) vs the “steam first” experience of the deck.
Dedicated builds for the Steam Deck and other handheld PCs are to come later this year.
What’s the use case here? Why chose this over SteamOS?
I think they want to be more appealing to hardware vendors. SteamOS has shown that there’s a lot of value in a handheld offering a polished end user experience that windows handhelds aren’t able to match. The goal here is probably to partner with other handheld PCs makers to offer a customized Linux based option that has out of the box support for all of the popular PC games stores.