Yes, my editor was quite concerned while I was writing CINDER EDNA that I not even use Disney's CINDERELLA as a jumping off place. In other words, I had to be sure that all the humor, ideas, jokes, images, etc. were based on Perrault's version of the story, which is in the public domain.
Disney is VERY litigious. They once sued the owner of a childcare facility for painting pictures of Mickey Mouse on the walls--I kid you not. They have an entire department of lawyers ready to defend their copyright. Publishers all know about this.
BUT what people are telling you is true--you absolutely can write your own book on the topic. And even more so if yours is nonfiction and theirs is fiction. No one stopped writing about elephants after Disney produced Dumbo. No one stopped writing about lions after they did the Lion King. It's quite doable. Go for it.