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.

Well, maintaining a distro specific to your hardware is one thing, supporting it for others - that's a huge undertaking.