Not true.

He can’t prevent anyone that received the code under the GPL from using (and distributing it) under the old license. He also can’t relicense code that he received under the GPL only under the new license.

If he receives a new license from the other contributors to distribute under a more restrictive license, he can do that because he has a dual license to the code and is not relying on the GPL for his right to distribute.

That would be true for competent web developers. Unfortunately, those are a vanishingly small subset.

Probably because of this. The commenter overstated the situation, but there are valid and serious criticisms to be made.