Only a few lines later we have the first paragraph of the GNU GPL, which includes the sentence "Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed."

IANAL, but I would guess that that implies that the author is in violation of the GPL for adding that restriction?

The license doesn't apply to the copyright owner afaik

I believe the GPL itself is also copyrighted, and its use is covered by terms.

Adding such a restrction does not have to change GPL itself, it’s just another sentence applied as part of the license by the author.

The lines were added to the top of the license. Changes don't need to go into the middle of the file to count as a modification.

I don't see it as funny - the author is probably mentally ill and suffering.

Actualy he seems to have common sense