Earlier quoted context omitted.
Of course, the copyright holder can license as they wish. But the quoted terms of the GPL are in the license that the author is distributing with the software, so we can also follow the terms of the GPL and remove the extra restriction they just added. The author is trying to do contradictory things: add extra restrictions, but release under the terms of a license that allow us to remove those extra restrictions. If…
If I have a license that says "you may use this, you may not use this", then can people use it? Honest question, I don't know how self-contradictory licenses work. Do people get to pick and choose what they want to follow, or does the whole thing become invalid?
You may also use software without a license, if you don't get caught.