I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by so

I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by some tens of centimetres. But just from common sense, it must be several orders of magnitude less than that, because otherwise your compass wouldn't work near the ocean. In practice, you need a very sensitive instrument to even measure the relative permeability of water, which already means that it's completely negligible for the purposes of this discussion. Wikipedia quotes water's permeability as 0.999992. By that measure, an eighty-times stronger argument against the researchers' due diligence would be "how do they know the cable isn't actually carrying 1599 amps? Did they measure the current?"