Yes, if they wrote it, they own the copyright (unless they have assigned that copyright to someone else, of course, like their employer). The GPL confers extra rights to non-copyright-holders over and above what copyright law gives them.
The copyright owner is never technically in violation of the GPL since they have all the rights they need under copyright law.
What they can't do is to pull back the code already released under GPL, that would be covered by estoppel (basically: if someone does something based on a promise you made, you can't later withdraw that promise - it's an equitable thing).
But they can re-licence their code as much as they want, selling it commercially or even not releasing later versions at all.
Keep in mind I am not a lawyer (though I deal with them quite a bit), I am certainly not your lawyer, and this advice is worth every cent you paid for it (which is zero).