There is no such thing as a perfect amp and the review even explains the SET compromises.
I've been told the most important characteristic of an audio system is linearity and SETs are inherently non linear due to the topology of SET circuitry. SETs amplify the top half of a wave form more than the bottom half. And by the way this is as much the cause of their high 2nd harmonic distortion as the low negative feedback.
This is not perfection. It's an orange compared to an apple.