So in the most well known vintage idlers (Garrard and Lenco), the motor drives a separate and discrete idler wheel via friction between a driven shaft and the idler wheel. The idler wheel in turn contacts either the inner rim of the platter (Garrard) or the under horizontal surface of the platter (Lenco) and drives the platter via friction. The Black Onyx is more like a formalized version of adding a Teres Verus or VPI rim-drive motor to an existing belt drive. In those set-ups the wheel that drives the platter is an integral part of the shaft of the motor. There is only one interface that depends upon friction, between the wheel and the platter. I guess this is why TT calls it "direct idler drive".
Just to generate a discussion, I would guess that there may be some advantage to the Garrard/Lenco idlers in that the separate and discrete idler wheel provides an additional amount of isolation between platter and motor. On the other hand, the motor in the TT is likely to be so hugely advanced in terms of noise, vibration, and speed constancy over either the Garrard or Lenco motors, that this small advantage is swamped out.
Just to generate a discussion, I would guess that there may be some advantage to the Garrard/Lenco idlers in that the separate and discrete idler wheel provides an additional amount of isolation between platter and motor. On the other hand, the motor in the TT is likely to be so hugely advanced in terms of noise, vibration, and speed constancy over either the Garrard or Lenco motors, that this small advantage is swamped out.