How can a three-phase motor have 20 Nm of torque at 1432 RPM and 1.8 kW?
20 Nm of torque does not coincide with maximum RPM so you can't just multiply that torque and maximum (unloaded) radians per second because that would be nonesense.

The manufacturer is most likely telling you that the motor can produce a maximum torque of 20 Nm at a significantly lower speed.

Maximum torque will be when the motor is significantly loaded and turning at 859.4 RPM or 60.0% of full (unloaded) speed.