It's not true. You don't need "authorization" to create a derivative work. You do need a license to distribute copyrighted works, including derivatives. And this only matters when you are distributing it to a sizable audience.

For example, if you rent a movie, you can watch it with your family. Nobody is going to sue you for distributing the movie with 5 people in your room. That's pure nonsense. Same with music, books, etc.

If you try to play the movie in an establishment with dozens of people, then it can become a problem, because you're essentially a theater now.

I'm not a lawyer so I don't know what the law is on selling fan art on a convention or even privately commissioned fan artwork. But things aren't as draconian as people assume it is.