A) Wasn't the article suggesting that would be 4-bits end-to-end in this hypothetical photonic matrix multiplication co-processor? ie. the weights are 4-bits

B) Power consumption and speed. Essentially chips are limited by the high resistance (hence heat loss) of the semiconductor. Photonics can encode multidimensionally, and data processing is as fast as the input light signal can be modulated and the output light signal can be interpreted. I guess this would favour heavy computations that require small inputs and outputs, because eventually you're bottlenecked by conventional chips.