This is, fundamentally, an episode about life's most existential questions. Some of those questions are fairly simple: what is good TV? How many shows about the same true crime murder is too many shows?
Some of those questions are much more complicated, like "if a time traveler poops in the woods and Steven Moffat is the showrunner and he changes the rules of how the story works so that detached parts of the time traveler's body follow him around through time and space ... what are the implications for Thanksgiving?"