You're right, either umpire can call a balk.
But, there is absolutely no such thing as the highlighted. No umpire school, CCA or NFHS mechanics teach "above/below" the belt for balk responsibilities. If you see a balk, you call a balk, period.
The statement was also made that "Good coaches gain every advantage they can." That is true, that's why the umpires are out there, because coaches will cheat if they can.