@ScanX As with many Google software/service launches, I imagine it is at least to some extent a matter of trying to manage the variables in play. If this follows the typical pattern it will only be a Pixel exclusive for a short while before rolling out more broadly.
Got it working using the chrome browser on my Galaxy S9+ with a PS4 controller connected via bluetooth. You just have to make sure that desktop view is enabled in the browser options settings.
Basically it's for quality control purposes, however I own a Pixel 4 XL and I can say that the quality is awesome. it's super fast and I'm able to use my PS4, Xbox one, and Stadia controller with no input lag at all.