Does it? I think this pop-science article overly simplifies antibodies and immunity...
I'm no doctor myself, but as was explained to me by my doctor friends (who however are not specialists in this subtopic either), there's many different layers to the immune system. It's entirely possible to have antibodies and/or antibody-producing capacity in your body, yet not be "immune" - if by "immune" we mean that you cannot get the illness, both in the sense that you don't get ill, and in the sense that you don't become asymptomatic carrier (i.e. the virus starts replicating in your body, but not enough to make you feel "ill" effects) (both of which are kind-of important if you want to stop a pandemic). Long-term immune system "memory cells" (B-cells) might still need time to be activate and start producing antibodies, so you can get infected, ill & a spreader, but not seriously ill because your immune system re-awakens and starts producing antibodies and you recover much quicker than you otherwise would. (This whole mechanism is the reason why you need booster shots of some vaccines.)
This is, of course, based mainly on experience from non-coronavirus like viruses. This one might be different.