Yes, that's true. So, why do I say it's a hot energy herb?
Sometimes the Chinese will express an herb's energy (hot or cold) by the outcome or reaction to that herb. Whereas, in western herbology, we can identify the energy of the herb itself.
Using a hot energy herb such as senna can be like fighting fire with fire. It's considered a counter-irritant. Therefore, when you take senna, it can clear a hot condition with the outcome being cooling. That's why the Chinese call it "cooling" due to the final outcome.