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.