If someone at the time of death has high measured oxygen saturation (eg 98%) is it still possible that they died of asphyxiation? In other words, does death via asphyxiation always kill by reducing oxygen saturation?
Yes, it is absolutely possible. There are several poisons, most notably cyanide, which interfere with the body's ability to take up or use oxygen from the blood, despite it being present in ample amounts to otherwise support life. This is known as histotoxic hypoxia.
As an example, a telltale sign of cyanide poisoning is a flushing of the skin, turning deep red over time. This is caused by veins, which typically carry oxygen-deficient blood, carrying oxygen-rich blood as the cells are unable to use the oxygen before arteries become veins at the capillaries. Cyanide interferes with one of the enzymes, cytochrome c oxidase, involved in the electron transport chain and ultimately responsible for producing ATP. Without this step in the electron transport chain, ATP production slows to a halt, and so does the consumption of oxygen, so many victims of cyanide poisoning have close to 100% blood oxygen saturation at their time of death.