This goes to the question of epistemology. How do we know what we know? In this case, the linkage was established statistically by contact tracing. In addition to the 7 deaths, there were also 39 cases at a nursing home where 6 of the 7 deaths also occurred. When they do contact tracing on infected patients they find the linkages to the wedding in question. It may not be 100% certain that all of them contracted the disease from that wedding, but the chances are extremely slim that so many contact tracings lead back to the same event unless there was a substantial causal relationship.