They trademarked that version of Mickey. Only the cartoon itself and its copyrighted IP are public domain. The issue is that you can't use the copyrighted material without using the trademarked Steamboat Willie Mickey design. There was a similar, lesser version of this problem when Action Comics #1-6 reverted to the estates of the creators. While they could have used anything in those 6 issues and DC hadn't trademarked that image of Superman, the S shield we all know was trademarked, and a lot of familiar material was still under copyright, making their IP functionally useless. Disney's game is tighter. Because Steamboat Willie's design is trademarked, just distributing that short could be grounds for market confusion. You would be distributing something prominently featuring a trademark for Disney, which could lead consumers to think you are Disney.

As I understand it, you can get around that by clearly stating up from that you are not Disney nor are you affiliated with them in any way.

Very likely. People making creative works derivative of Steamboat Willie that are obviously different are also probably going to be fine. If your version of the Steamboat Willie design is unique, you're good. You can Five Nights at Freddy's that mouse and if you're backed by a decent studio willing to take on that project, you can pull it off.

Isn't the areoplane Micky also now Public Domain?
The first thing I did Jan morn was copy-paste a pic of that into a pic with the World Trade Center. That way I got to combine Mickey with the whole Ai paranoia.
It's funny reading as there was a write-up on the "Horrors" that could be produced by Ai and this was one.

https://insidethemagic.net/2023/10/mickey-mouse-9-11-lobotomizes-ai-nk1/?fbclid=IwAR1GjbXgBov6hbmquevCE_F26nX5UFWq4_WReofcn3KP1KHaA3rhAsSSawg

Only the silent versions of Plane Crazy and The Gallopin' Gaucho are now public domain. The sound versions will be public domain next year.