That line about update arch base means it is shipping only a year old packages and not 1.5 year old ones.
As long as Valve provides more snapshot updates in the next six months, I don't see the problem. The ~~TiVo~~ toy is maintaining currency.
I'll concede this might be annoying for the subset that chooses to use a Steam Machine as a server or development workspace, once the product releases. My condolences to this imaginary and small group. They'll be shocked to find immutability, too.
Ask anyone outside of Valve or strange hobbies what packages actually 'make' SteamOS; they couldn't tell you. It's simply not their concern.
In fewer words: if you need/want to know the lifecycle, you'd know. The "by the way, they're a year old" message that started this thread is in bad faith. The Fedora release that equates to any EL derivative is comparatively ancient. It's fine.