Sure, but that’s unlikely, given the wording. “Owner of the software” is fairly clear
The article's text said, "Winamp will remain the owner of the software". That does not, in fact, preclude giving it a FOSS license, nor does retaining a related trademark. GP was correct. They can make it FOSS and keep the trademark and copyright. I don't see any reason to think it unlikely.
The creator doesn’t “surrender” their copyright, but someone can fork it and then have ownership of their version
Forking someone's copyrighted work does not change ownership of the rights in any jurisdiction that I know of. If you meant "ownership" in a difference sense, like maybe control over a derivative project's direction, then I think choosing a different word would have made your meaning more clear.