Paraphrasing some tweets on the matter earlier, the result is rather underwhelming because it plays on a discrepancy between what they mean by quantum supremacy (QS) and what people tend to think QS means.
What I find most people think QS is supposed to mean, and what I assumed it meant until a month or so ago, was that there exists a computable problem (in the CTT sense of computation) and an actual quantum computer, such that, at some scales, the problem is tractable on the quantum computer but intractable on all classical computers.
The problem the Google QC folks have demonstrated is not computation in the CTT sense. It is a physical process of sampling that involves computations as part of the process, and as with any physical process, it can be simulated approximately by computation. They have good reason to believe (proof? I'm not sure but it should reasonably be assumed true by default anyway) that computation to similate the process is going to be intractably slow. This is not surprising at all. It's a fundamental consequence of quantum mechanics that lots of physical processes will have that property.
That's not to say it's entirely uninteresting. There are likely useful applications of the sampling problem they implemented, and as I understand it, it provides examples of large classes of physical systems which are not amenable to efficient computational simulation. But it has nothing to do with whether or how soon a QC will be able to compete with a (classical, CTT) computer solving computable problems.