Earlier quoted context omitted.
Disney was actually founded on Alice in Wonderland, Mickey Mouse was created years later. At the time, Alice in Wonderland would have still been copyrighted if it were under the copyright regime Disney etc. instituted since. The corporation pulled the ladder up after it. Incidentally, Mickey Mouse was in many ways a ripoff of Universal's Oswald the Rabbit.
Incorrect: Mickey Mouse was created in the 20s, Disney's Alice in Wonderland was released in 1951. You're right that the book would still have been copyrighted. Oswald the Rabbit was also created by Walt Disney, in 1927, and sold to Universal. He then created Mickey Mouse as a replacement character for his own movie.
These weren't really retellings of the original stories, but it would nevertheless not be hard to imagine they would count as a derivative work under modern law and would thus have been illegal to produce without a licensing deal.