Connected device compatibility is discussed here (including the M1 versus any connected device besides Apple TV and Roku):
Given the yo-yo-like nature of various Slingbox models' feature sets, this sort of thing happens.
...and even though the chart in the link shows the Apple TV is supported by the Pro-HD, it isn't really:
"The Slingbox PRO-HD and the Slingbox SOLO are both capable of streaming audio and video through AirPlay (non-mirroring) to Apple TV. However, due to a hardware limitation, the aspect ratio may be skewed. Later Slingbox models, such as the Slingbox 500 and the Slingbox 350, do not have this limitation."
So a warning to all future purchasers......