This header seems incompatible with your reading: "...it means only GPLv2 with
the possibility to relicense under GPLv3". As I understand the header,
it says that
the licensee (a person to whom a license is granted) can himself select which
version of the license to apply; the licensee does not need to ask the authors
of the code to grant him GPL V3, as that is what they have already done.

Best regards,
Dan