You are correct that Nemeth arrows are not pointing devices. The example you cite shows the result of a mathematical process. It's a little like balancing the equations and formulas in chemistry. The math process changes the original equation to the one that follows the arrow.
The BANA math committee has posted an update on the BANA website on the application of Formats to Nemeth transcriptions. Regarding arrows used in grammar:
17.12: For print arrows showing progression in word formation use Nemeth arrows. These arrows would not have to be listed on the special symbols page. If doing a literary transcription, of course you would use whatever is specified in Braille Formats. Even though word formation would probably never occur in a technical book it is still in the realm of "process", so does not violate the "no pointing" restriction.
If this is no help, please let me know. Dorothy