Totally totally untrue. I don't trust him and that statement is one of the reasons why. It is an out and out lie.
It is absolutely true that Jeremy manipulates people. It is also true though that such people allow themselves to be manipulated.
Manipulating someone into sending the threatening letter would not really help him though so I doubt he is behind it. It would not help him because it has little chance of actually getting Harrison to abandon the project.
While people claim some guilter would do it they have nothing to gain either. Only some wacko supporter could think that it could benefit trying to silence Harrison and be crazy enough to think the threat could work or did it just to annoy him. Otherwise it was someone who wanted to help Harrison by giving him press attention. Those are the realistic choices someone looking to help get him press coverage or a wacko Jeremy supporter trying to annoy him or scare him.
After seeing Jackie's irraitonal behavior I can see why some would suspect her. She is pretty wacky and defends Jeremy despite no evne knowing or caring about the evidence and facts but I have no idea if she would be wacky enough to try to annoy him and get back at him by sending such a stupid letter or not. I don't know enough about her to try making a judgment either way.
Some crazy lady got stuck in a chimney trying to breka into a house of a guy she met online and had a couple of dates with. I bet no one who knew her predicted she would do anything that stupid or crazy. People who are emotional do all sorts of stupid things. I have no idea how many passionate wacko supporters Jeremy has. So no idea how big the pool of suspects could be. It doesn't have to be a long term supporter it could be someone who just got involved recently. The chiney wacko didn't date him much or know him long.
As others pointed out stamps and envelopes are no longer licked and even when they were careful people used water, they had sponges especially for the task. At any rate that only works if you have the DNA of the culpret on file or get a warrant for same. So you need probable cause anyway.
Fingerprints also need to be on file and often are hard to lift from paper and can be avoided. The chance of ever finding out is nil unless the person does something else.