It would be important to look at the sentence immediately before the one quoted to gain understanding; "Other courts have held that evidence of third-party threats are admissible to support a claim of self-defense if there is also evidence from which the fact finder may find that the defendant reasonably connected the victim with those threats."
In reading the filings and Supreme Court Decision it does not appear that the defendants attorney did enough if anything at all that would "reasonably" tie the victim to the treats. So without making a ruling on the admissibility of third party threats even making room that they would be admissible, the evidence that was being put forth would not be because there had not been a connection made between the threats and the victim. Just having a relationship with a group of people does not make you party to everything that group does, especially if you did not participate.
I am not a lawyer, had a chance to play one on stage once though.
LawyerJan, welcome to the forum. Discussion on these topics is good, and we can use someone with some credits to keep us in line at times (man, can we come up with some strange interpretations).