Watching the Minecraft is Fun video, it is easy to see that the two children are each individually immersed in the game. However, there is a connection , with the older brother helping the younger sister and connecting within the game. They do not see it as playing to win, but rather, as a collaborative, fun game, helping one another to move on in their ‘learning’ and their understandings.
It was interesting to read the course notes about each working on their own autotelic creativity. I had never heard of this term before, even though I have read about Mihaly Csikszentmihalyi and in particular his concept of the experience of ‘flow’. Wikipedia notes, that autotelic “describes people who are internally driven, and as such may exhibit a sense of purpose and curiosity” which is different from external motivations e.g. comfort, money, power, or fame.
“An autotelic person needs few material possessions and little entertainment, comfort, power, or fame because so much of what he or she does is already rewarding. Because such persons experience flow in work, in family life, when interacting with people, when eating, even when alone with nothing to do, they are less dependent on the external rewards that keep others motivated to go on with a life composed of routines. They are more autonomous and independent because they cannot be as easily manipulated with threats or rewards from the outside. At the same time, they are more involved with everything around them because they are fully immersed in the current of life.” 
The next video to reflect upon was the Mario Universe Hunger Games. Apparently “this video and its creator are part of the popular culture emerging from games” (course notes). I think the audience that the host is aiming to attract are other players of this game, in particular younger players, but players who have developed play game relationships e.g. the presenter talks about his sister. The assumed knowledge needed to follow the presenters narrative is the basic rule of playing Minecraft, and finding different elements within the game. I think this is definitely a niche audience, with tacit knowledge are the assumed knowledge elements, but also the rules and ludics (mechanics) of the game such as time limitations etc.