> Conventional wisdom is that distributed consensus is not possible at this kind of performance

I'm not sure why you would think that? If you can assume the fiber is the same in both directions you know the round trip time is exactly double the latency of the connection. Then you know to phase shift your start time by that much when you get a start signal and you're in sync.

Obviously it's not trivial in practice, but it's not a fundamentally insurmountable problem.