It is the same con as £500 RCA cables ("interconnects"), gold plated mains plugs, capacitors with gold plated terminals, etc.
This utter crap goes back decades. In late 1970s I was building power amps, 400W per channel, 0.01% THD at 400W RMS into 8 ohms. It was not difficult. Not cheap, with pricey RCA 2N5239 transistors (10 of them per channel), etc.
Then, early 1980s, the CD arrived with its 16 bit linear PCM, and that was obviously the end of how far you could take sound reproduction for the benefit of the human ear. Speakers remained the weakest link and still are; nothing anybody can do. But the industry had to carry on somehow... gold plated mains plugs came next. Then big DAC boxes for CD players which oversampled many times, etc. Then valve amps which sound "nice" because the 5-10% distortion is mostly even harmonics.
This Russ Andrews thing takes it to new levels though, with added boards which do nothing at all.