It's interesting because of the scaling law. No matter how much acceleration matrix multiplication gets on an electronic circuit, its energy usage is always going to scale as O(n^2.something). The implication here is that the energy usage by doing it optically is O(1). At least, that's how I read "We found that the optical energy per multiply-accumulate (MAC) scales as 1/d where d is the Transformer width". The best you can hope for is to stay on the right side of the constant factors (which, currently, the GPU world is).