I am curious as to why the microcut is "hrx only"
Your HRR is the K8 version of that model and came stock with stacked, QuadraCut twin blades.
Last January, Honda updated the HRR to a K9 version, and besides adding the Clip Director control, all HRR models now feature offset twin MicroCut blades. Yes, it's true the HRX blades physically "fit" on an HRR, but they've not been tested or approved for use on the mower. Honda recommends only using blades specifically engineered, tested, and approved for each model.
Unofficially, I understand you can update your older K8 mower with new K9 MicroCut blades, but just be sure to get the ones for the HRR, not the HRX. They look the same, but aren't. I know this for a fact because it was discussed in great detail why we could not make one set of blades fit both groups of mowers, and it is for rigidity, strength, horsepower/torque ratings, etc. that are different enough between the HRX and HRR engines and deck designs that require a slightly different blades to be needed for each one.
Here are the part numbers and retail price for MicroCut blades (now standard on all HRR models) that may be installed on any HRR:
Blade, Rotary (lower):
72511-VL0-S00, $13.12
Blade, Rotary (upper):
72531-VH7-000, $12.98