Assuming you mean ATV3 (ATV4 has the Sling TV app, so AirPlay really wouldn't be needed). Yes, ATV3 won't AirPlay Sling TV. They (Sling TV) are aware of this and on their website they say they're working on fixing this to enable legacy ATV3 viewing via AirPlay
However, what works well is use the mirroring option on your iPad or iPhone to your ATV3. Also set overscan to off in your ATV3 settings, and Sling TV will come in perfectly on your TV.