Provided that the developer can either remove contributed code or seek copyright licences from every contributor it can be done. Whether this did happen or not is unclear, the developer appears a little arrogant about having written the majority of the code, showing little appreciation towards minor contributors. We cannot tell whether the contributors gave permission for their code to be re-licenced or if their code was removed.

Personally, when contributing to GPL projects I would expect that this kind of thing wouldn’t be possible. Using the GPL is a very philosophical choice of licence and is a move to say that you really care about your users