This joke will only be funny to you if you've ever taken a physics class. But making apparently oversimplifying assumptions in mathematical models of physical phenomena usually works pretty well. The fourth coauthor on this paper, Marc Parmentier, is a theoretical geophysicist who was one of my thesis advisors at Brown. He was famous for such statements, while deriving or solving equations, as "well, pi squared is about ten, so we can cancel that..." or, even worse, "pi is about two..." -- generally, geophysicists make apparently ridiculous simplifications, because all they're really interested in is working things out in as much detail as is necessary to see whether their model produces a result that has the same sign as reality -- for instance, they'd like to see a mathematical model for Mars' crust make the north look low and the south look high, without much regard to how low or high. And if the model gives a result that's within the same order of magnitude as reality (that is, within a factor of 10), they're pretty happy with themselves. But, once in a while, someone comes along and shows that it's worth it to make your model just a little bit more complicated.

The assumption of a nonuniform temperature across the core-mantle boundary does some other neat things besides explaining why the south is magnetized but the north is not. Stanley and coauthors go on in their paper to show that since this magnetic field doesn't have the same shape as the magnetic field we're used to on Earth, studies that try to reconstruct where Mars' rotational axis may have been in the past are flawed.