HomeKit /AirPlay support is in latest Roku os. I use AirPlay all the time but not homekit yet. I don’t think it’s dependent on any model types but I’m not certain on that. That being said I think the Roku ultra model is worth the extra bucks. Better wifi signal and nicer remote (enhanced voice remote) plus a USB port for external disk drive.
Roku Ultra 4660X RokuOS 10.5.0 build 4208 (was somehow downgraded from 4210 thank goodness as cleared up many issues!) Sony Bravia 55A8G TV