From your description it sounds like you are confusing the functions of the idle air screw with the idle mixture knob.
The knob that was spinning is the idle mixture knob and is used to control fuel delivered at idle, while the idle air screw on the inlet manifold is used to control idle AIR .
If the knob was spinning then it was probably just stuck in the "adjust" position (spring compressed) and has now freed itself.
As long as you didn't touch the spinning knob she should be fine.