Valve said they want to partner with companies to support it on other devices. Guess they haven't taken up Valve's offer for whatever reason. They should also have more faith on just how well proton works.
the problem is, supporting more than just 1 hardware spec is very difficult. im not just talking about their SOC, but the controller parts, the networking devices, all of the things that can be more temperamental on linux than it is on windows. and that's without considering the long list of games that don't work, and probably never will, for various reasons out of valves control.
You'd have to be utterly mad to think driver support is easier on Linux. Most of the time it's either it exists or it doesn't. And since obscure hardware is generally moved by interest, the answer to issues is usually "Why don't you code it?"
This is a huge step from the drivers just straight up existing at all on windows. Usually the drivers that compete/surpass windows are for both popular projects, and projects where the information is generally available (hence why Radeon card drivers are fairly good, and Nvidia has been an ongoing battle)
This is all to say that yes, having a company backing the hardware's drivers helps a lot, so the point of supporting more than 1 hardware spec is a very good point.
Most of the time it's either it exists or it doesn't.
That actually sounds easy. Specially when you are the vendor and can choose what to bring to the device.
The nice thing is that, while Windows has a poor record at keeping compatibility each generation and requiring new drivers and maintenance provided by third parties (which doesn't sound "easy" for them), Linux keeps very stable and has accumulated an astonishing library of hardware support spanning decades.
It's amazing how I can plug almost any printer in a Linux machine and have it work out of the box, no extra drivers required, no nothing.
The most likely reason why Lenovo is choosing Windows is because they already have an ecosystem of crappy Windows software they typically preinstall in their machines and they probably expect they will be able to bring their own "special sauce" to hook some customers in... rather than just shipping with a more generic SteamOS that they wouldn't have as much control over & wouldn't give them an edge over competitors.
I don't think anyone who daily drives Linux understands how annoying getting virtually anything to run properly on that platform is, if you can get it to run at all. It doesn't support most keyboard/mouse software, it doesn't support any anticheat for any major games, and it's chronic use of CLI means that it can never get a large enough amount of users to become a viable platform. The only way that Linux users will get platform support is if someone actually makes a windows level user experience, and Ubuntu is still nowhere near that bar all these years later. If I can't use the things I use everyday on windows, on Linux without hitting the command line once, it's not happening for the general public.