While it's true in his category of "PhD's that work" the most useful degree is Computer Science as it's closest to "data scientist" of all the available PhDs yo

While it's true in his category of "PhD's that work" the most useful degree is Computer Science as it's closest to "data scientist" of all the available PhDs you still need domain knowledge to apply it. So while it's no coincidence that Physics, Computer Science and Stats/Maths postgraduates are all in high demand in the field of applied economics itself (hedge funds, high frequency traders and the like) it's also notable they aren't just trading by themselves.