I think your colleague may have been confused. The swab tests for a current viral infection and not for antibodies, which is a different test. The more recent (fast) swab tests look for antigens which are surface proteins of the virus itself, i think, and are also not the same as antibodies which are produced by us in response to the infection and need to be tested with a blood sample.
But the swab test I think is not infallible and may throw up false positives and negatives, although I don't know what percentage accuracy they are estimated to have.