I also would be very interested in finding out what his source of that information was. AFAIK, there was no autopsy performed and it was widely publicised that Dr. Atkins slipped on ice while walking to work, hitting his head and suffering a subdural hematoma which he did not recover from. Unfortunately, with such an injury, the odds were not with him as it is fatal in 90% of elderly and young who sustain such an injury.
Sounds to me that this doctor is repeating rumor and not fact and I would take it as such unless he can produce a credible source (ie actual medical records) that says otherwise. Obtaining such information would be very difficult since HIPAA laws prevent such medical information from being released to the public without written consent from the family.
It could also be possible that this doctor is confusing the cardiac arrest that Dr. Atkins suffered approximately a year prior to his death with his actual cause of death, but at that time an angiogram was performed and showed no blockages present. The cardiac arrest was caused by cardiomyopathy that Dr. Atkins had for several years and developed as a result of a virus that he picked up while travelling overseas and that information was released to the public by Dr. Atkins and his family in response to the rumor that he had suffered a heart attack (MI).