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At least the abstract says something different:

> Nevertheless, rare but recurring RBD-specific antibodies with potent antiviral activity were found in all individuals tested, suggesting that a vaccine designed to elicit such antibodies could be broadly effective.

Very low levels, but still produced. (Note that I still have to dig through the paper.)

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My understanding is that those RBD-specific antibodies are not necessarily produced as a response to covid19, but I could be mistaken.

In addition:

> Plasmas collected an average of 39 days after the onset of symptoms had variable half-maximal neutralizing titers ranging from undetectable in 33% to below 1:1000 in 79%

Of course this is only a single study and more research is needed, but I notice people draw way too strong conclusions from studies that show low seropositivity prevalence.

It looks like another preprint found antibodies in the airways mucosa instead of blood. That would perhaps explain why some were undetectable.

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