Incorrect. The triac varies POWER, by chopping out part of the wave. For the first half of the cycle, the peak voltage is unchanged. What you read on a multimeter is the meter's impression of what it is seeing. A universal motor will change speed when the power supplied varies, no matter how that power is varied, voltage or pulse width, or portion of a sine wave. AND the TS has mentioned washer machine motor a few times, and that would never be a universal motor.Apologies - That was a bit of an oversimplification.
Universal motors: speed proportional to voltage, so the triac dimmer varies the voltage, and thus the speed.
Induction motor runs at synchronous speed minus slip.
Reducing the voltage reduces the current, and hence the torque and allows more slip.
Provided that the torque:speed curve is monotonic then it does work. For a fan, power = speed cubed; but for other types of load can be rather less successful.