If you touch type then learn the OS X shortcuts of CTRL-B, CTRL-F, CTRL-P, CTRL-N, far more efficient than moving your hand over to the arrow keys and works in (virtually) all places you are editing text.
But focusing the menu bar with Ctrl-F2 does indeed allow me to use Ctrl-f and Ctrl-b. And after hitting Return to open a menu, I can use Ctrl-n and Ctrl-p to navigate down and up.
Interesting.
I will have to try more, but I still think that there are places in macOS where Ctrl-b and Ctrl-f do not work, and I have to use the arrow keys, instead.