That is the kind of study they did, yes. And the results are consistent with commonly accepted timeline, not with the "I had Covid in October 2019 but they covered it up" crackpots.

The prevalence they found is extremely low, less than 0.05%, despite their samples going all the way back to Mid-March. Since they did not test on any samples that are definitely pre-Covid (e.g. early 2019, late 2018), we can't calibrate their false positive rate. But if we assume the specificity of the tests was 99% which seems on the high end for antibody tests, and that the false positives are uncorrelated, we're already in the region of false positives feasibly explaining literally all of their samples.

The data is just totally inconsistent with any kind of widespread transmission of Covid in the US in 2019.