They are not the sole copyright holder, there were many contributions under the AGPL license. Unless there was an explicit copyright assignment signed by each of them then they are also copyright holders and would have to approve any change in license.
The other contributors were mentioned in the issue before it was sterilized:
As covered elsewhere, they're not the sole copyright holder.
It is not so simple I'm afraid; he allowed outside contribution from people who thought they were contributing to free software. He deserves the backlash. If he wanted to make money, he could have created a sub-project using octotree binaries and extended modules under new licensing terms.