My understanding is that similar to the notation on the chart that "DME/DME RNP-0.3 NA, the helicopter notation (Helicopter Visibility Reduction less than 3/4 SM NA) is just a charting convention.
Even though it's obvious from the (non-helicopter) 5 mile min vis (reduced to 2.5 miles for helicopters), the issue of 3/4 SM is never relevant. The same goes for the "DME/DME RNP-0.3 NA, which is irrelevant also since in order to fly a RNP -0.3 approach (which an LNAV approach is), you must use a properly certified GPS system to achieve the RNP -0.3 performance accuracy (DME/DME RNP can never be used). I think both are just a charting convention.
This question has been asked before on another forum (can't remember which one) and this seemed to be the accepted most reasonable answer.