I think there’s one key thing you missed: you have never bought a copy of the game on Steam! It’s always been a license. Valve simply made the fact clear now because of legal changes.
so the next question, is this retroactive
So the answer for this is a solid no.
Usually I sympathize with sentiments like this (“people use X because of uncontrolled circumstances”), but browsers are not one of them.
If you have a website that requires the use of Chrome, then just use Chrome for that website! It’s not an either-or thing – you can install both browsers and use Firefox as the primary one.
And that’s what makes this statement so problematic. You don’t earn anything by staying exclusively on Chrome, when both it and Firefox can work alongside each other.