I, too, am Japanese, was born, raised and live in Japan, by the way.
It is a fact that "it would be done by the same hand because the style of writing could only be done by a skilled hand" is wrong, and the flag in this thread is most likely a fake, anyway.
Tell him/her my words.
能筆家が親族や同僚の代筆をするのは至極ありふれたことでしょうが、全署名が同一の筆跡である場合はその寄せ書きが偽造であることの有力な論拠になりうる、というのは論を俟たないと思われますが、いかがですか。