I believe that is only recommended when using an analog source since the DAC2 has a hybrid volume control (analog pot for analog sources and digital volume control for digital sources). The analog pot will be massively inferior to the digital volume control in tracking and noise, and channel imbalances will be greatest at either end of the volume spectrum, hence the recommendation to keep the analog pot above 11:00. When using a digital source and the digital volume control then the position of the volume knob should be irrelevant; tracking should essentially be perfect and contribute no noise at any position.
Tracking may not be a problem, but as you reduce a digital volume control, SNR is reduced. The DAC2 has a very high SNR, but you still want to keep it above 11 o'clock for "ideal" performance.