Yes it has to be set to 6 channels if that's what the processor downstream accepts. What the statement is saying is that 8 channels are sent to the HD621 via HDMI and 6 are output. At least that's how I read it. this was apparently Text from Meridian during the launch.

Living Room Kit: 861v4+ID40, DSP7000, DSP5500, DSP5500HC, HD621
Bedroom Kit: F80
QNAP TVS-471, Roon, iPad Air2