The D rate is limited by the 50watt discharge limitation of the charger. From your manual "Note that depending on the voltage of the pack, the current you set may not be obtainable if the total Volts multiplied by Current (A) exceeds 50W." There may be a linkage from the LIPO Max screen choice which is set at 2C by default.

I don't have that charger, but there may be other limits imposed if you're on AC power. Try setting the default Max C higher and see if that lets you set the D rate higher as well. With 2S lipo, you shouldn't be bumping into 50watt based current limiting, but it is limited to 5amps regardless.

That charger is built to charge other chemistries, NiMh are discharged for maintenance and they are cycled as well typically after not being used for a while to bring the mah back up. Lipo doesn't need discharging (seems to be a waste) except for storage purposes and that can be slow. I tend to discharge in an RC if I need too, its a lot faster.