Finally we're seeing another handheld vendor jump in with Linux. The AYANEO NEXT LITE was announced today and much like the Steam Deck, they plan to ship it with SteamOS!
I expect this is simply a case of "Valve Time" on that effort. Perhaps there's a long-term path towards more "official" SteamOS on these devices, but if there's any area where HoloISO diverges technically from SteamOS in a way that's not reconcilable, that'll be problematic for offering users a "seamless upgrade".
Long-term, I think it would be slightly more harmful to Valve's efforts if more manufacturers started standardizing around HoloISO, so I expect that this might be a motivating factor to speed up their efforts to bring official SteamOS to third-party devices.
I think HoloISO is a good target for compatibility. Valve hasn't released a generic ISO for steam OS yet but much of their efforts have made their way upstream. They've then landed into other projects like chimera and bazzite. Since this device hardware targets HoloISO it should mean that it will be easier to install other operating systems on it and get full functionality.