Yes it is true, certainly for the purposes of this discussion.
The only way you as an author don't have any copyright in a work is if you write it for someone else (eg your employer) - like technical writers, many engineers, journalists, ghostwriters and most screenwriters do. Otherwise you as the author always own the copyright - as the article you linked to makes clear.
No serious publisher would ever try to get copyright from an author. If the publisher you are talking to wants the copyright to your work then it is time to walk away.
Stallone has never owned any share in the rights to the character - he may have created the character and written the story, but he sold the rights to someone else. Before we waste too much sympathy on him we should note that he's made nearly $450 million from those films despite not owning any rights. Most actors are lucky to earn a tenth of that in their whole careers.One very well known incident of losing rights is Sylvester Stallone and his Rocky creation.
Why Sylvester Stallone Doesn't Have any 'Rocky' Ownership
It is something we as authors have to watch when we sell the film and TV rights to our books, because what can happen is that the producers want to be able to write their own stories using the characters and setting we have created. At that point you, the author, risk losing creative control. As I wrote earlier, get a good lawyer or agent if you get an offer to buy rights to your work.