If there are two identical loops, why does the GPU one have a higher TDP rating than the CPU one?

Both cooling loops are identical in every way, it is just the mounting to CPU/GPU that has a different mechanism. The performance difference is because for the GPU, we have direct contact with the die. If we removed the IHS on the CPU, we could achieve similar performance levels to the GPU.

For reference, a single side (or single loop with evaporator and condenser) can handle 350W, so if both are operating under ideal conditions, the case could handle 700W in total but we dont expect people to de-lid to achieve this.