Do they use a product? WordPress’ license is GPL. However unfair it might feel they can use it under the terms of the license. Matt has chose the license and has to live with that choice. He’s doing basically the same thing Redis/Elastic Search/Mongo did but worse. He wants the money without getting the relicensing backlash. Which, I’d argue, is even worse than those other instances.
The reality is that copyright law treats the author, not maintainer, as owner. Matt is the author of less than 1% of the codebase by line. To relicense, he would need consent from the remaining 99%. I'm not sure how you could argue otherwise.
Yes, if everyone signed a CLA with copyright assignment, as is common in some projects that are still nevertheless GPL, then he could relicense, but because he has not done that (and because it's actually a fork initially), he cannot relicense.
