Yes, that’s correct.
The correct voltage setting will depend upon the gain that you need across the entire system in order to secure an appropriate volume from your loudspeakers. If it is still too loud wherever you set your preamp volume control except if set excessively low then the output voltage of the Bartok has been set too high ( e.g. 6v). If no matter how high you set the preamp volume control it just isn’t loud enough then the voltage output has been set too low ( e.g. 0.2V).
For historic reasons ( the specification of the CD replay system), in most circumstances a voltage output that is 2V given full modulation of the digital signal ( 0.0dBfs) will be found to be appropriate. Of course there are circumstances where this is not the case, hence dCS’ decision to allow the setting of other voltages.
Although control of volume will now ideally be via the preamp ( to preserve maximum resolution) this does not necessarily mean that the digital control on the Mosaic app is totally redundant. For example I listen a lot to BBC Radio 3 which is a classical music station that makes a wide dynamic range available. The result is that the mean modulation level is low compared to some other stations, hence my preamp’s volume level is set high. When I switch from it to BBC Radio 4 to hear the news ( low dynamic range), I still use the Mosaic volume control to rapidly reduce the level below 0.0dB. The preamp volume control is some feet away from my listening position and I don’t want to blast myself out of the room! Of course in this case it is not particularly SQ sensitive but it is handy.