From the link you provided:
This test, however, revealed that the D/AC-1100 truncates incoming 20- bit data to 16-bits. I routinely use 20-bit resolution test signals now that processors are available which will pass 20-bit data to the DACs. Feeding the D/AC 1100 with a 20-bit dithered source from the Audio Precision System One Dual Domain resulted in the four Least Significant Bits (LSBs) being truncated, and with them, the dither. The result was the appearance of a lot of harmonic distortion because of the lack of dither. When I saw the high distortion, I repeated the test with a 16-bit test signal to create the plot of fig.3. Note that even though the Crystal CS8412 input receiver, PMD100 digital filter, and PCM63 DACs will all operate on 20- bit data or more, the D/AC-1100’s internal architecture provides only 16-bit pathways.