I am not a teacher.
It does not mean "more based on truth".
You are seeing a relatively new definition of "about", an extension of the meaning we see in "a story about fishing". We can say that one thing is about another meaning that the second thing inspired the first. For example, "her dress was all about showing off her figure" or "the Christmas party was about building team spirit." It still sounds slangy to me, and I think it is still informal. It is often "all about".
To break down the sentence: "This" refers to Mr. Geller's mood. His mood is "about" something in the way described above. It is about "the fact that once you make the decision, itís like acknowledging Nanaís actually gone." His bad feelings about Nana's death might have inspired his present mood.
Student or Learner