I think that "Exactly what has happened since and the precise cause of Mr Bennett's death is not clear." indicates that the reporter is not clear, not the physicians. Other sources like https://www.wbaltv.com/article/man-received-historic-pig-hea... talk about infections and indicate his illnesses and treatment were well understood. Even though the patient was a public figure he has HIPAA rights and the hospital has obligations towards him. I don't doubt much more will come out in subsequent published research.

The chance they don't understand in great, great detail the cause of Mr. Bennett's death is nearly zero. The chance that IRBs aren't equipped to handle the ethics of this decision in the future are also very low.