When he reported on it before the alleged attack happened, it could always have been called of for one reason or another. I don't know how likely or possible such an effort would be, but at least on a small scale it checks out: If I wanted to poison someone, and someone credible enough to be believed in stated that I was going to do just that, who's to say that I can't just change my mind at that very moment? Without finding the poison it would be nigh impossible to prove.

This is something that will always be problematic when reporting on something that hasn't happened yet. As the future hasn't been written, there's always room for all actors to adapt and change their plans.