Our “closest ally,” folks!

"Low confidence" may refer to the level of corroborating evidence or available data. If there's insufficient time to obtain corroborating evidence a "low confidence" assessment can be reached.
"Low confidence" doesn't refer to the probability that something may or not happen.
Glenn Greenwald knows this, and the authors at WaPo know this.
Should the Secret Service, who is tasked with protecting the President but not WaPo 'journalists', ignore any CIA "low confidence" assessments regarding an assassination attempt on POTUS ?
You didn't include this from the WaPo article ...
"Ultimately, the White House and the Secret Service appear to have decided they could take no chances."
Would your preference have been 'we decided to take a chance, and we were wrong' ?