How does a solar PV inverter throttle the input power
As described in this answer the output current of solar panels is directly related to the output voltage. If you short circuit the panels you will get a very high current - but basically no power, because the voltage of the panales drops towards 0 V. The same is true for a open circuit: In this case the voltage is pretty high, but there is no current flowing. Again, with P = I * U the power output of the panels is very low.

A solar inverter is using a switching DC/DC regulator at the input to controll voltage and current. In the typical use case this circuit would be controlled to find the point of maximum power (the MPP). But when there is a derating, because the inverter is not able to supply as much power as would be possible, it can increase its current draw from the panels and by that automatically decresing the voltage up to the point, where the desired power output is achieved.

In this diagram you can also see the power over panel voltage and current:

Image from Analog.com