It has a core truth to it being "most" amplifier work better if they are loaded less.
This alone is a very generalizing statement but saying 1/10 is the ideal is an over generalization.
It may be true that some signal is Attenuated coming in the Preamp if it has less input impedance.
But you know what is also Attenuated. Noise.
The Signal can easily be amplified in the source by the designer/manufacturer this resulting in better signal to noise.
The Lower the Input (and output) resistance the lower the noise can be and the better the signal to noise can be (for a given signal level)
Manufactures are already hitting practical and almost physical limits of achievable better SNR with "normal" Hifi impedance and voltage Levels.
A ways around this limits:
Extreme cooling ( i'm waiting for preamps with integrated refrigerator)
Lower Impedance (input as well as output).
higher voltages.
It is no accident some high end preamps have lower then usual input impedance.
So is low input impedance. overloading the output opamp of the DAC?
Since
there is no output opamp in this DAC i don't thinks so.
Could be its output performance decreases with lower impedance load...
But at least provide a hypothesis why.