Since this isn't explicitly clarified in the article, it's worth mentioning that THD (or THD+N for that matter) is not always a helpful spec to compare two amps. You can easily have two amps where the one with the worse THD is actually the superior one because its distortion is in low-order even harmonics, while the nominally lower THD amp has more distortion in odd or higher-order harmonics. The distribution of distortion over harmonics is as important as the raw sum. (For the layperson who doesn't care about the math: this is why tube amps sound good even when their nominal THD spec may be higher than low-end solid-state amps.)
If your amplifier's distortion is less than 0.1%, you probably won't hear it, whatever it is.
The main reason people like the sound of tube amps (the "warmth") is due to the fact that most of them can't amplify high audio frequencies very well.
For reference, the frequency range of human hearing is taken to be 20 Hz to 20,000 Hz. Hearing is logarithmic in frequency, i.e. the perceived difference in pitch between 100 Hz and 200 Hz is the same as between 10,000 Hz and 20,000 Hz - an octave.
Tube amps have output transformers. It's difficult to design wideband power transformers (heck, designing any transformer is difficult), so most of them have reduced output in the top octave. (AKA "high frequency rolloff".)
One reason for the bad reputation of transistor amps is this: -
Many early transistor amps (and some still today) were prone to oscillation at MHz frequencies under some circumstances. The oscillation itself is inaudible but it affects the linearity control of the amplifier (negative feedback). Amplifiers were oscillating at MHz frequencies for a millisecond or two and losing control, which had a noticeable effect on the sound.
The oscillation wasn't picked up by the designers (of the early transistor amplifiers), because who needs a 100MHz oscilloscope for designing audio circuits that only go up to 20kHz? That's just silly. (Until, of course, it isn't.) And in those days, 100MHz scopes were expensive.
There are other reasons. For example, tube amps withstand some overload; transistor ones don't. Exceed the output transistors' second breakdown rating for a tenth of a second, and suddenly you have no sound. Your new speakers you just brought home from the store killed your amp. "But they're 8 ohms, same as the old ones!" (Another long story there.)
It took a while to figure out all this stuff and how to protect against it, and in the meantime, reputations were made and lost. Tube good, transistor bad.
