The FAA sent alerts to airlines after the Horizon Air incident saying “a validated jump seat passenger attempted to disable aircraft engines while at cruise altitude by deploying the engine fire suppressions system.”
This is what I was looking for. From the title it seems like he just tried to throttle down but he was looking to incapacitate the engines. If he was successful it would have meant the airplane would have been in a glide without power. It seems likely as a pilot he’d know when to do this in a flight too to remove good chances of a safe landing. Truly terrifying!