The journalist spoke with two workers within the production facility. "One of the workers said a supervisor confirmed two positive cases to a group at the Fremont-based seat assembly facility" This is an example of just one supervisor confirming two positive cases. It is likely that there are many more.
Furthermore, "But because Tesla restarted production a week earlier, there could have been cases that were never reported to the county because Tesla was “not required to directly report known cases” before the agreement, county officials said."