It's an Associated Press (AP) article. You can find it in literally dozens of papers online, including the example you cited. It's the same article reproduced over and over.
Note that Northern California papers that have been doing their own reporting on this story--such as the San Francisco Chronicle and the Fresno Bee--have not said that heat has been ruled out.
I think it's likely that the author of the AP story just made a mistake.
It has to be a mistake. There's been nothing from the sheriff or local papers saying heat was ruled out.