I disagree with that assertion. It is simply not a high-fidelity amplifier, unless you're driving speakers that are resistive loads, because its frequency response will track the impedance curve of the driven loudspeaker. A high-fidelity amplifier will have flat frequency response regardless of the driven load.
That may be so, but the little DTA still sounds good for its price and what it was intended for. I have the 100a and it has tight powerful bass and airy sound with a wide sound stage. I paid $59 on sale and it was the best $59 I have spent. Connected to a good source and a pair of Monitor Audi S1's it sounds pretty good. Certainly not hifi but definitely mid fi.
Can it hold a candle next to my Pass amp, BAT preamp and Magnepan speakers, no! But it is certainly enjoyable to listen too and that's what it is all about.