There is a precedent for moving two pieces on the same move....
I hate whomever decided that the variation at the beginning of this post was playable, and whomever revised the rules to account for it.
Sir. Please be aware that the correct form in this case is "whoever" and not "whomever."
mate ends the game, and you can't claim an illegal move once the game is over.
Perhaps there is an omission on the manuscript. The end looks like the classic Scholars mate where 3. Qf3 a6 (or whatever) then white gives the killer blow with 4. Qxf7#The B on c4 covers the Q
There is a precedent for moving two pieces on the same move....