While it's true that not everyone participates or uses Steam, Steam can still report that it's running on a Linux desktop. The client can report what platform the browser is running on when you open store pages, and they can definitely see your platform when you wishlist something.
In addition, it can tell if it's running in Wine, because Wine reports itself as the platform to Windows apps.
Wayland support is now on the way, and NVIDIA is bringing RT and DLSS support to Linux. DXVK/Proton support is also improving, and NVDIA has been supporting virtual machine use since last year. They're also contributing a bit more to the Nouveau open-source drivers.
Still, it's mostly behind what AMD has been doing, even though performance is more or less at parity level compared to Windows.
Steam is actually much higher.
In April 2016, Valve announced that they had around 125 million users.
https://store.steampowered.com/news/21534/
Valve now uses the term "lifetime active accounts". Lifetime active means that the account was used for at least one purchase, with actual money (and currently that's the small $5 that you need to spend to enable community features). Charts that Valve produced in 2017 at the Casual Connect conference said they expect 1.5 million new customers per month.
https://www.geekwire.com/2017/valve-reveals-steams-monthly-active-user-count-game-sales-region/
Tracking from that month to today, we have 62 months of growth, which totals about 93 million new accounts, giving Valve over 218 million accounts. Let's call it 230 million to make room for the smurf accounts used for CSGO and money laundering. Back in 2017, more than half of all Steam account owners were playing games in any given month.
In 2016, Valve's peak was 14 million active players at any point. In 2020, they were over 15 million and consistently butting against the 20 million mark. This week they're averaging 24 million players at peak.
Valve likely added almost 50% to their subscriber count in the last two years thanks to the pandemic, so my numbers are likely out by 30% or more.
We're casually looking at around 260 million lifetime active accounts on Steam. Valve is almost 40% of the PC market. It's only going to reduce from now on as Xbox Game Pass steals more market share, but Valve is a significant chunk of the market.