Puzzling Class is not abstract and does not override abstract method

Question

This question has been asked many times, and usually I know how to handle it. This time OpenJDK 16, Netbeans 13 and Gradle are puzzling me. Although I am convinced I am overriding the necessary method the compiler insists that I am not.

I am trying to extend GHObject, even writing the method manually, although usually I use Netbeans' hints to implement the missing methods. Also removing the @override does not help.

This is what I came up with as a minimum example:

package settlers.installer.model;

import java.io.IOException;
import java.net.URL;
import org.kohsuke.github.GHObject;

public class MyGHObject extends GHObject {

    @Override
    public URL getHtmlUrl() throws IOException {
        return null;
    }
    
}

and the compiler emits

error: MyGHObject is not abstract and does not override abstract method getHtmlUrl() in GHObject
public class MyGHObject extends GHObject {
       ^

To rule out problems in Netbeans and OpenJDK 16 I switched to running command-line Gradle and openJDK 19. No change.

What could be missing?