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AirPlay mirroring is different, it's a system level thing. Try any app were a dev has blocked video out and standard airplay, AirPlay mirroring still works, it doesn't use video out like normal, it mirrors the devices screen, this why the mirrored screen doesn't use the whole TV screen. That hows a dev cannot block it.*
Not true. The Sky Go app, for example, still doesn't work when you use AirPlay mirroring. The actual message on the TV screen is something like "HDMI Out is Not Available When Using this App."