True, but as you say, even canon can be ignored or rewritten. Look at Dallas, which retconned a whole season into a dream in order to justify bringing back a killed-off cast member. And any long-running canon has subtler changes in it as well. Sometimes rather glaring changes are made without justification; in The Greatest American Hero, the main character Ralph Hinkley's surname was abruptly changed to Henley for a while after John Hinckley's attempted assassination of President Reagan, while in Alien Nation, a deadly disease contracted by one of the main characters in the pilot was completely abandoned as a plot point by the second episode and never mentioned again. And that's nothing compared to the Alien Nation revival movies. The first one retconned the entire series finale out of canon, and later ones retconned the family's baby daughter into nonexistence.
So calling something canonical is by no means a promise that it will always be obeyed. As I said, it's a myth that canon is binding on the creators and restricts what they can do. The reality is that canon is what the creators say it is. And part and parcel of what that means is that if they say the canon is something different than it was a decade or a year or a week ago, then it becomes something different.
So yeah, you have a point that canon from creator-overseen books and comics is more easily abandoned than onscreen canon, but that doesn't prove it isn't canon, because even onscreen canon is changed or ignored all the time. Yes, it's only canon until the creators say it isn't, but the same goes for all other canon.
