It is an intriguing mystery.
I doubt that Cooper went to visit this lady and told her to shut up. That doesn’t make sense. Why would anyone do that, in person, no less.
I think the writer believes it was the FBI who visited the woman and told her to shut up. He says the FBI was concentrating on a different location and thinks this FBI visitor didn’t want her account to split the search effort because the FBI believed it was close to solving the case.
Article suggests it was an FBI agent, not Cooper, who visited “Janet” and told her to keep quiet.