Ding...Ding...Ding.....we have a winner!
DO we. Not one drop of brake fluid goes to the rear brake caliper from the front reservoir. Bleeding/flushing the rear SYSTEM per the steps outlined by either members here or the service manual will replace the old fluid with the new fluid you pour into the rear reservoir.
The OP was asking for a video on the rear brake flush. He got a really good link from Joe on a how to. Not once was it ever stated there was air.
The brake fluids from either the front or rear reservoirs do not mix/link. If you bleed/flush either system per procedure, you have replace the old with new through each reservoir. The rear caliper is operated from the front caliper via a mechanical (SMS) operation. That fluid in the SMS comes from the rear reservoir. The operation of the front calipers from the rear brake pedal is done through brake lines from the rear master cylinder. There are delay valves and proportional valves that control this linking, but no fluids are shared.
Is it best to do both, YES. Will there be a catastrophic failure if you only do one or the other, no. Now, if you are having symptoms of air in the lines, it would be hard to tell which circuit to do. Then by all means, you do want to do both.
If all you are wanting to do is replace old with new, then you can do one or both. I will agree that it makes sense to do both. But to say you can not just do a "rear brake flush" is not correct. I am not a Honda trained mechanic. Would be interesting to get their take on it. I just may do that. I know one that I would trust my bike with, which translate to my life.