This isn't really what your Twitter link says. That links says: "Both IgM & IgG (those are two different kinds of antibodies, with IgA being a third) were low or undetectable at day 0, but increased by day 5 in nearly all patients (N=16)"

-- Reports strongly indicate there are many infected people who are asymptomatic after five days.

Which is to say, this test could be extremely useful if applied widely and systematically to many people; food service workers, health care workers and so-forth.

Currently the virus test has a week turn around time. So both tests effectively find people with a week's exposure.