AFAIK this only concerns casting TO a chromecast (or miracast) enabled device, which has been possible for a while now (the new part being the support for it in this desktop application), but you can’t make a device a chromecast receiver because that part is proprietary. You can set up your rpi as a miracast receiver or use sunshine/moonlight to stream your whole phone screen / pc monitor to your rpi, but the ability to just press “cast” on your phone and then do whatever else on it while the content plays on your TV is sadly locked behind the proprietary walls of google / apple