In a fictional universe a medium (book, movie etc.) being "canon" is the original author's seal of approval claiming "this happened". I think this is a joking

In a fictional universe a medium (book, movie etc.) being "canon" is the original author's seal of approval claiming "this happened". I think this is a joking extrapolation to the real life, and the "author" here is, in best case real life, or as an approximation, historians' position based on evidence.