Let us suppose the three WIV got sick with COVID-19.
This was not reported because unlike previous leaks, they were working on off-the-books gain of function research for the PLA using a novel DNA sequence, and the virus sequence was not in public databases.
The three were treated in the hospital and luckily infected no one else in the hospital or among close contacts, which caused the Chinese authorities to become overconfident that this strain is not very infectious (no human-to-human infection). As scientists know, no matter how you change the sequence of a virus, you cannot predict how infectious it is. However, unluckily, one or more of the workers traveled along the subway line connecting WIV to the Huanan Seafood market. At the seafood market a superspreader event occured.
When the Huanan Seafood Market outbreak occurred, Chinese authorities delayed because they mistakenly believed it was not very infectious due to the 3 workers not infecting anyone in the hospital or among close contacts. By the time they realized their mistake it was already mid-January.
Anyway, that is a plausible scenario for the WIV spill theory.