Neither.
In my opinion, in order for a product to be "semi-canon",the work would have to be approved or accepted by its canon source's creator(s) despite that the work was not created by them. In this case, The Lion King: Six New Adventures is not semi-canon because it was not approved or accepted by the original filmmakers going by the audio commentary, where they referred the cub at the end as "Fluffy".
And, the books aren't (profit) fanfiction, in my opinion, either. Why? Because the books weren't necessarily written by "fans". The writers of Six New Adventures were just freelance children's book writers that wrote tie-in books with other studios.
For example, Judy Katschke's (who wrote "Vulture Shock") resume at Jacketflap.com states she wrote tie-in books for movies produced by DreamWorks Animation and Columbia Pictures.
So, in my opinion, TLK:6NA is just unofficial/non-official, and exists in its own universe. Even, an article on The Lion King Wiki on what's unofficial in TLK universe states so...but it's written by fans, too.