I do not know enough Japanese to say for sure, but it looks like the primary (and only) definition for 密告 is not right. The definition is “secret information.” However, this neither fits the provided example sentence, not does match the primary definition in other dictionaries.
Based on other dictionaries [1, 2], the primary definition seems to be “anonymous report” or “tip-off.” JMdict [2] does list “(secret) information” as a secondary meaning. I believe it is referring to the leaked information in the anonymous report itself. However, the term “secret information” is incredibly misleading in English as it carries no nuances of a tip-off.
I believe the primary definition should either be “anonymous report” or “tip-off.” Can someone more familiar confirm or correct me please?