This is not true unless the throttling algorithm is so broken that it's oscillating between extremes.

The parts have a curve of clock speed versus voltage. More clock speed means higher performance. That goes further up the voltage curve, meaning more power.

Throttling just moves the card further down the voltage to clock speed curve. It reduces clock speed, reducing performance.

The cards don't "perform faster by running slower". If you run the card slower, it performs slower.