It would depend on how the tracing was done I'd expect. I believe it is often possible to sequence the data from the infections to provide a link between them, that may have been done in this case. (https://twitter.com/nextstrain - is an example)
https://www.cdc.gov/mmwr/volumes/69/wr/mm6945a5.htm
It looks like just contact tracing here, not genome sequencing.
The patient in question is A2 and the study does not say whether they ever tested positive or had symptoms.