In general, when a politician cites the existence of secret evidence against an organization they're hostile to, it means the "evidence" is one of: (a) fabricated, (b) too weak to withstand public scrutiny, or (c) or gathered illegally in a manner that would cause outrage if admitted (eg. by secret police infiltrators in a manner that broke the law).
Remember, DARVO and outright lying aren't only used by fascists: all authoritarians do that.