Not clear if test was for active virus or immunity.
Seems an important distinction...
All they test for is antibodies in your blood. Antibodies mean your body has been infected with the virus and reacted to it, possibly even gotten over it.
A swab would be gor active virus
It was testing to see if they either had it at the time or had had it. Many people had had it and didn’t even know it.
Antibody testing, probably.