I know this doesn't solve your problem, but AirPlay hasn't been disabled in the app. I've successfully streamed from my iPad Pro to both a 3rd generation Apple TV and an LG TV with AirPlay built in.
For some reason or another, probably money, data mining or both, HBO doesn’t want you casting your iPad stream on to a big screen. The good news is there should be corresponding AppleTV app for HBO that will get you what you want.