I am pretty sure that Blizzard have literally zero need to watch a stream of a world first mythic kill, when in their own 'behind the scenes' videos they have out-and-out stated that they watch the events live: as in really LIVE, they are watching what is going on in that instance server in real time. I believe it was in their 'Looking For Group' documentary that they showed and talked about the tech that let's them do it.