I thought that copyright holders could relicense code however they wanted? I don't think GPL or not is the issue, but whether or not all third-party contributors have assigned the copyright of their contributions to the party trying to relicense. My understanding is that this is often difficult or even impossible in practice to obtain after the fact codebases with large numbers of contributors over the years if signing something ahead of time wasnt't a requirement previously, and I don't have any insight into whether Wordpress is in this situation or not, but I don't think whether the code is GPL or not is relevant to this
Yes, the copyright holders can relicense, but its highly unlikely they will do this to enable a fork to have a different license, and wordpress started as a fork of b2.
If Mullenweg had been the original developer it would be valid criticism.