Play Anywhere is not possible as its coming to Steam. Play Anywhere is exclusive to Xbox1 & Win 10 ONLY. When they choose to release on Steam it cannot be Play Anywhere. So thank Steam release for that otherwise it would be free to Game Pass for very little money per month
MS can always eventually choose to put these on Game Pass though. That is what they do monthly on Xbox1 and Game Pass proper is coming to PC this year according to Phil Spencer so I still think they might end up on it as a Game Pass sub seller!
Tbh its great these games are coming to PC but lets face it most of them will just be the same dated gameplay with recentish graphics. I can see why they are releasing in reverse order leading up to the next proper Halo game coming to PC as well.