That's partially true, but Valve's long term goal with SteamOS is to break their reliance on windows.

Windows previously had plans to require all new software to be installed through their Windows store, which would have destroyed Steam. Thankfully Windows backed off from this plan, but they are instead pushing windows S mode for new computers. If you buy a budget PC it will probably come locked down where it can only install Windows apps. This can be disabled, but requires you to have administrative privileges on the computer (which many kids/etc won't have). It also requires you to log into the windows store with a Microsoft account to disable it, so they require you to take all the steps to install windows store content before you can choose to not use their store.

It's not going to happen overnight, but I suspect the long term windows plan is to force more people (especially kids) into getting software and games through the Microsoft store. People like to buy games from the same place, so if you get kids started building their PC gaming library in the Windows store they'll be more likely to stick with it even once they have other options.

So companies selling windows handhelds will still help valve out short term, but their long term goal (and the reason they invest so much into Linux) is to no longer be dependent on Microsoft windows to sell games. It's also why they're pushing for Steam on Chromebooks, to both get young users invested in steam and as another windows alternative.