They don't need to do anything the GPL says if they're the sole copyright owners. People are bound by the GPL because copyright law normally prevents them from distributing works without the author's permission, and the GPL is the only thing that allows them to do that.
The GPL itself even points this out explicitly in section 9:
> You are not required to accept this License in order to receive or run a copy of the Program. [...] However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.
Another way to look at it is this: the only people able to take them to court for violating the GPL are themselves.