I am guessing its a typo and maybe he meant 935mV. I've tried different voltages and I think its impossible to be able to run it that low and even get Windows to load.

He might get lucky and have an extremely high ASIC quality however its not possible that 635mV would be enough to mine with.

With the RX 470/570, they generally need about 120 Watts (from the wall, not including cpu+ram+mobo) to run without hardware error or hangs. If you want it to use less power you need to mine an algo like XMR which uses like  100 Watts.