The main benefit optical interconnects aim to provide is not about the speed with which photons propagate, which as you point out is not all that much faster th

The main benefit optical interconnects aim to provide is not about the speed with which photons propagate, which as you point out is not all that much faster than electrons in copper, but it is about bandwidth. Propagation speed of course affects latency, which as you point out is already running up against relativistic limits, but the physics of photonics vs. electronics affects the maximum bandwidth achievable in a modulated digital signal. At a high level, one issue is that the capacitance and inductance present in copper limit the ability of discrete pulses to remain discrete voltage levels that still are distinguishable between logical high and logical low when modulated at a very high frequency due to the capacitance of the wire which effectively blurs the pulses together as they propagate. The other fundamental issue with electronic interconnects is the issue of shot noise, which for information theoretic reasons can further limit the theoretical maximum bandwidth logical high/low voltages in copper. Photonics does not have these limitations, but as you say, is still a very long way off from practical application. It is important to point out, though, that the motivation (unlike the article's slightly clickbaity title) is not speed but bandwidth.