It is technically libellous of Jeremy, but the problem he faces is that as someone convicted of mass murder he is deemed not to have a character to defame. He would therefore not get anywhere.
I think the journalist was obliged to invite me to comment if he intended to use my name in the article. He clearly intended to use my name as it featured prominently in the letter and I can understand that he needed to find out what my views were. To say I was shocked is an understatement.
I see two problems for suing for libel.
1) as you identified he has no good name to ruin so peopel are free to essentially make up anythign they want wih impunity
2) Even if he had some image left to ruin, the bar for proving a lie for a public figure is pretty high. Unless it ocudl be shown the reporter sent it himself or knew for sure who did then there is a reasonable basis to believe Jerey is behind it based on the content.
If someone sent a threat in my name and the person receiving it claimed I threatend them they had a factual basis to believe I threatened them. If an investigation proved I didn't send such threat and at that point they still accused me then at that point they have no real defense.
An ambiguous letter with an ambiguous threat would be much easier to sue over if someone decided to attribute it to someone without having any evidence to support it. An overt threat in contrast provides its own attribution.
Some people on the planet are actually dumb enough to make overt threats like this but I don't believe Jeremy is one of them. That doesn't deprive someone receiving such threat of having the right to believing it was from him though. Only clear proof someone else did it would eliminate such right.