From the post-mortem, it doesn't sound like like the problem was a single network cable. Redundant network switches have existed for a long time, and they're certainly using them (but not bothering to mention it in the post-mortem).

Their problem was that they only have two transit providers, and one of them black-holed about 3-5% of the internet. Since it was a routing issue, I'd guess it was either a misconfiguration, or that the traffic is being split across dozens of paths, and one path had a correlated failure.