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Linux gaming is getting faster because Windows APIs are becoming Linux kernel features
(www.xda-developers.com)
Unlike in Windows, in Linux the graphics UI concerns are outside the kernel, so graphics layers sit pretty close to the hardware, so even just from that higher performance was already expected as the adaptor layers such as Wine/Proton improved because the kernel itself is faster and gets less in the way.
Then, of course, the Linux kernel tends to be developed by people with lots of experience, overseen by people genuinelly at the Software Architecture level of experience and given its architecture benefits from know-how both from the server-side and the front-end sides of software development.
My point being that the Linux architecture is not only far more mature and controlled by far more experienced people than the Windows one, but it also gets way less in the way of graphics applications trying to squeeze as much performance as possible from the system hence in Linux improving such graphics applications or the adapter layers for them can go further in delivering better performance than in Windows were the kernel becomes a performance bottleneck sooner.
What we're seeing now is those two effects delivering, especially once the Wine/Proton adaptor layers matured and entered a stage of more performance and stability improvements than feature implementations.