I had it working with the Edge 830 yesterday.
IIIRC, you have to turn the Humon on with one press - red LED. Then press the button rapidly twice. It should then flash blue. Almost immediately, the 830 began showing SmO2 in the Humon data field I downloaded from ConnectIQ. There was no calibrate prompt. Perhaps the double click assumes you’re holding still for it to calibrate.
Interestingly, when I downloaded the .fit file from the Garmin into GoldenCheetah, it showed all the values GoldenCheetah supports for the Moxy: SmO2 and tHb too. Even though Hemoglobin is not displayed on Garmin, the Humon is sending these data.