Question: What percentage of windows gamers are actually Linux gamers who are using wine/proton or some anti-cheat workaround/VM that causes them to be reported as using windows when they are in fact running linux?
Using proton, I regularly see crash reports where games want to report that I was running some version of windows, which is a result of how proton implements wine. I never send such reports as they are of little use to developers.
Proton is a modified version of Wine. If the underlying game is sending a bug/crash report, it may "see" itself as running under windows as that is what wine presents.
"Proton is a tool for use with the Steam client which allows games which are exclusive to Windows to run on the Linux operating system. It uses Wine to facilitate this."
Correct. Steam, a native Linux app, will run Windows games via Proton. As Steam itself, which is collecting the data in TFA, is a Linux app, it shouldn't be affected by thinking it's running under Wine/Proton. As to your original question, I don't think many developers release information on what OSes their players are using, so I don't think we can tell
Using proton, I regularly see crash reports where games want to report that I was running some version of windows, which is a result of how proton implements wine. I never send such reports as they are of little use to developers.