Does the quote above suggest Alberto (AGD) is misleading in his saying his GANFET is made for audio?
No- not in my opinion. IMO it suggests he had a batch made that have low output capacitance. Low output capacitance reduces ringing in the output section (which is also caused by circuit board trace inductances and power supply issues- they can be in parallel with the output capacitance of the device which results in ringing and noise) as it switches and thus helps the devices to run cooler. The ringing can be controlled by a snubber filter but the output devices will still run hotter. So a low output capacitance is desirable.
In practice we found no need for a snubber circuit as there was no detectable ringing. IMO GaNFETs allow for lower noise designs. I think this is very important as switching noise and parasitics can mess with digital equipment even it the amp meets EU emissions directives. So you want to be many dB below the maximum allowed by most government entities. We found that many tube amplifiers radiate more noise than our class D.