A: Traditionally, “concerted” has meant done in concert—that is, jointly.
However, the adjective had an earlier meaning of organized, coordinated, or united.
How is that a "however?" Those are basically synonymous. Coordinating and uniting both require more than one entity. I guess you can say one person's effort can be organized, though.
And since the 19th century people have used “concerted” without any collective sense to mean purposeful and determined.
It still feels wrong to me, but I'm literal-minded.