Before the GraphicsHertz() was added, there was a GetGraphics() command that retrieved several of the graphic parameters including the refreshrate -- so it did work in DX mode... But when that command was removed and replaced with several stand-alone ones, this ability broke.
The command for grabbing all parameters is GetGraphicsMode, which is still available.
However, I find that on my Mac laptop the hertz setting comes back as zero. Works fine under Windows and Linux no matter whether I use DirectX or OpenGL. Seems to be the opposite of what everyone else is experiencing!