I'm not sure how Valve is seen to forfeit any Windows related profit.

They are still thoroughly supporting Windows. A Windows gaming system will have Steam on it, and most gamers still prefer Steam while on Windows.

When Windows 8 happened with the Microsoft store, Valve saw the writing on the wall for the eventual problems they would face, and did SteamOs and SteamBoxes. However, not much skin off their back, as they didn't "bet the company" or anything. It then pretty much let those efforts die off when the Microsoft Store wasn't quite the imminent existential threat it looked to be. However, the Xbox-ification of the Windows ecosystem may prove to be a more imminent and dire threat now that Microsoft realized that "hey, we actually do have a gaming brand that enjoys some popularity and is basically just a Windows box already".

So Valve saw that the Nintendo Switch was such a hit and extrapolated to PC space. They could have had a horribly awkward device running Windows, which has forever sucked at serving this form factor and is not even vaguely amenable to 'total controller control'. However they decided to revive the SteamOS efforts since it was moderately close to enable them to actually deliver a pad-first UI for a handheld, with Valve branding front and center rather than Microsoft.

So the closest I can see to that claim is that Steam Deck opted out of supporting a handful of games (that also likely don't work well on the relatively low end specs anyway) rather than trying to make a Windows hand-held work against all the design points of Windows.