ESD strap grounding location
The ATX power supply earth pin you have circled is directly connected to the metal chassis of the power supply, which in turn is also directly connected to PC metal chassis.
So yes, connecting the clip to that pin is completely electrically equivalent of connecting the clip to unpainted PC chassis. There may be other downsides though, it may not stay clipped to the pin and it is on the far side of the area you are working. Surely there must be a better place to clip than the inlet earth pin, and you don't want to make extra scratches or grooves with the clip to the earth pin either, so that sharp edges don't wear out the mains cord contacts.