DD platters have more torque to maintain speed unlike some of the high end belt drives I've seen.
I wouldn't think torque would have anything to do with maintaining
a steady speed, that would be more a function of either the mass of the platter, or the speed of a feedback circuit (well, okay, more torque could respond to that feedback circuit faster...).
Torque has nothing to do with maintaining speed. Steady speed of the motor and flywheel mass of the platter are the determinants of speed stability. If anybody here remembers the original AR turntable, that thing had a pathetically low torque motor, yet it had some of the best wow and flutter / rumble numbers in the business.
The whole point of belt drive is to limit the potential sources of speed instability. A direct drive motor cogs, and with the servo feedback circuitry, it is always
going to be hunting for the correct speed, however little that variation is; that's the inescapable result of having a servo system in the first place. Those mechanical disturbances will
be picked up by the stylus, amplified and mixed with the music (ironically, its the high torque of the direct drive motor which generates more of this mechanical noise as it tries to correct itself).
With a belt drive turntable, there is really only one potential source of speed instability; the bearing on the platter. The motor is effectively isolated mechanically by virtue of the belt.
With a direct drive turntable, there are more potential sources of mechanical instability; there is not only the bearing on the platter (really the motor shaft), but there are the other issues of cogging and servo hunting mentioned above. There can also be magnetic interference transferred to the coils in the cartridge.
Its not just about speed stability; its the resulting electrical and mechanical disturbances being transmitted to the stylus.
Direct drive in turntables was really a gimmick introduced in Japanese turntables in the 1970s. Just like a lot of other such gimmicks meant to sell more gear with questionable utility, it was eventually found that turntable speed could be more stable with the benefits of belt isolation and a good motor. That is why its essentially impossible to find a high quality turntable which uses direct drive today.....even in Japan.