> a specialized low-power unit in an iGPU is going to be practically indistinguishable from a NPU, except that it will probably be easier to target from existing GPU API's.

Possibly, depending on how low the power actually is. We can't really tell from NVIDIA's tensor cores, because waking up an NVIDIA discrete GPU at all has a higher power cost than running an NPU. Intel's iGPUs have matrix units, but I'm not sure if they can match their NPU on power or performance.