Welcome to Yamaha Synth! Thanks for the question. You are correct.

Drum Kit Voices can be edited but they must be stored to the dedicated USER DRUM Bank... only "normal" Voices can be STORED to the Mixing Voice Bank.

MIXING and the MIXING VOICE.... from the Reference Manual...

A program in which multiple Voices are assigned to Parts for multi-timbral play in the Song and Pattern modes is referred to as a “Mixing.” Each Mixing can contain up to 16 parts. Each Mixing can be created by editing parameters unique to each Part and parameters common to all Parts in the Mixing mode.
Also, one Mixing has up to sixteen memory locations for saving Normal Voices. These Voices which are saved are called Mixing Voices. Normally, the Voice stored in the Voice mode is assigned to each Mixing Part. In this case, the sound of the Song/Pattern you created may change unexpectedly if you edit or delete the Voice used for the Song/Pattern Mixing in the Voice mode. Mixing Voices are provided to prevent these accidental sound changes.
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Drum Kits must be stored to one of 32 User Drum locations provided. Normal Voices contain typically one instrument sound made from 8 Elements, while a Drum Kit a Voice is typically made up of 73 separate instruments (one per Key). It's 73 Elements require special storage locations in a special Bank just for this type of Voice. Each Key in a drum Kit C0~C6 has its own Waveform, it's own tuning, it's own filter, it's own envelope generators, it's own routing to the Effects, etc., etc., etc.

Additionally, the Editing of the Drum Kit Voice must be done in Voice mode, if you are working with the front panel of the MOXF. If you are using the Editor you can edit drum while remaining in SONG or PATTERN mode. When you go to STORE your work, again, you must use one of 32 User Drum locations in the dedicated UDR bank.

Hope that helps.

Right, from my experience with Motif XF and XS, I knew drum voices were different. Just confirming you can't have drum voices as mixing voices, thanks