Clarkie
You appear to be making two statements:
1: High mass platters improve the speed stability of belt drives
2. The transmission in an idler is stiff, so the motor is tightly coupled to the platter.
and following them with a conclusion:
3. Therefore "the two systems are completely different".
The two statements are reasonably uncontroversial, but I cannot see how they are supposed to lead to your conclusion.
The obvious implication from your statements is that belt drives are necessarily less tightly coupled than idlers and that the platter in a belt drive is somehow free to rotate in an uncontrolled manner. This is supported by a statement in your previous post where you said that a belt drive has "a motor that only really pushes when the platter slows".
This is completely wrong. If the two systems are designed to have the same drive compliance then they are by definition equally tightly coupled. There is nothing to prevent this being achieved in practice; that it has not been seen as a desireable goal by the designers is an historical artefact, not a matter of physical necessity.
Axelwahl
you have confused the terms slip and creep in my post. They are not the same thing.
Mark Kelly