The issue, as it is presented in the article, is with how copyright law intermingles with trademark law. They noted that people will be able to distribute the original short. They also noted that any unique works that incorporate the iconic mouse (even in its original form) may run into litigation. Given Disney's financial interest in The Mouse, I suspect they were understating it.

It is easy to oversimplify what is going on here if it is only viewed through the lens of copyright. While it would be easy to dismiss trademarking a character in most fictional works as doing an end-run around copyright law, The Mouse appears to have been a fairly consistent and identifiable part of Disney's image for decades. In the casual meaning of the word, it has been a trademark of the company. There is also a question of how the evolution of the character plays a role. As far as I can tell, Steamboat Willy is dead an Mickey took his place. Disney's trademarking of Steamboat Willy can be construed as an attempt to do an end-run around copyright law, but the visually similar Mickey has had a more enduring (and endearing) history.

Or perhaps certain types of trademarks, in the legal sense, should have a limited duration as well. I don't know of many companies that have maintained an immutable corporate image across decades simply because it does not make sense. Culture changes, and companies should be adapting with the times.