It's that second question that might serve as a lead of sorts. How did they know the US had the facts, knowingly or not? They knew that that US intelligence had information about six sick lab researchers. How would they know that?

A related question is, how did US intelligence get the information that they failed to analyze? But the real intriguing point is that the source new more about the US intelligence than the US intelligence services themselves!

The simplest answer is that the source gave the information to intelligence previously, bundled with other intelligence, and the reference to the illnesses was simply glossed over and forgotten.