True, but if you’re not using that you can solder in your +12 V supply there (as long as you stay clear of the +5 V and +10.6 V outputs).
This post has a clear picture of the connector, btw:
Also, as for the 10.6 V reference we went over that in this thread and the MidiMuso design isn’t great if your +12 V supply is a +12.0 V supply. Sam ended up using a TL431 for the LMNC implementation, to make things more reliable if you’re using a typical modular supply (see long discussion in that thread).