By oxygen toxicity they are referring to excess depth on nitrox.
Nitrox standard mixes are 32 percent or 36 percent; max of 111 feet or 95 feet, respectively, beyond which the partial pressure of oxygen exceeds the safe limit. 160 feet is not an oxygen-toxic depth for air (21 recent oxygen) but their boat was known for providing divers with nitrox. For air the safe limit (the same partial pressure) is about 187 feet in salt water.
The fact they may have been exploring a cave may have made it more difficult to react quickly to early signs of oxygen toxicity - delaying any effort to start an immediate ascent. It’s also a possibility that others delayed their ascent trying to help the one that was incapacitated first.
We probably will never know.............