Tested by whom and how? If a trustworthy disinterested party positively identified the workers, took their blood samples, and tested them via a procedure with a low false-negative rate, then the test results are meaningful. Otherwise, there is reason for doubt.
1. The testing showing no Covid-19 antibodies was fraudulent and faked.
2. That testing was not faked. The workers did not have Covid-19 antibodies; their illness was caused by some other illness.
3. The report of the lab workers' illness is faked. (It came from a conservative newspaper, from unnamed officials citing an unnamed international partner -- where somewhere along that chain, someone had the proper motivation.)
I'm not arguing for any one of these things. We just honestly do not know.