The canon is just the main, ongoing story being told by the original creators or their direct inheritors
So Gene Roddenberry's novelization of TMP or Jeri Taylor's novels are canon by that definition? Taylor has implied her novels are, others implied they weren't... and it goes on.
So those creators and inheritors all have varying opinions on what is and isn't canon in Trek. So it comes back to opinion. Take what you want for your Trek experience or don't at your pleasure.