Heat stroke has not been ruled out as a cause of death. The only causes of death that have been ruled out are blunt force trauma and "chemical hazards" along the trail. Based on recent media reports, law enforcement is waiting for toxicology results before commenting further on a possible COD.
By the way, it's often difficult or impossible for an autopsy to confirm that heat stroke was the cause of death. “The autopsy findings of heat stroke may be minimal and are non-specific, particularly if the survival interval is short" (from
https://www.aaimedicine.org/journal-of-insurance-medicine/jim/2002/034-02-0114.pdf). When heat stroke is listed as an official cause of death, it's usually after all other possibilities are eliminated.
You mention that the official word is that these deaths are "unique" and "strange." Law enforcement used a lot of such language initially, but that began to change last week. In a statement on Thursday, the Mariposa County Sheriff’s Office continued to say that they were investigating various potential causes of death, but--for the first time--they focused more on the conditions that day:
"The sheriff’s office said the entire Savage-Lundy Trail loop, where the family was found dead, is approximately 8.5 miles, with 5 miles of that being a 'steep southern exposure path with little-to-no trees or shade' in the 2018 Ferguson Fire burn scar. Sheriff officials said temperatures there appeared to range from 103 and 109 in certain parts of that trail between 11:50 a.m. and 5:50 p.m. Aug. 15, the day the family is believed to have been hiking it."
Source:
https://www.fresnobee.com/news/local/article253770788.htm
I'm not suggesting that law enforcement has concluded that heat stroke was the COD, but I think they've shifted from not thinking about it at all to thinking that it's likely, or at least not unlikely.