Actually, this is very plausible. Pilots only talk when they have time. Smoke in the flight deck requires O2 masks. Talking to ATC is the least of your worries. Sometimes 5 or 10 minutes after an emergency procedure. ValueJet cargo fire gave the crew only minutes to response time.
Not buying it. Not issuing a mayday would in itself be negligent. If conditions were that bad that a ditch in the ocean was eminent, they could have given some indication that they were doomed.
Think about it. The article states that someone had methodically turned of several navigation aids. And I don’t think turning off a transponder would be protocol. And then reprogrammed the computer to execute a turn.
With two pilots on board, a mayday call would be doable. I would not be surprised it these jets had a switch one could hit in an emergency to automatically send a distress signal.
It would also explain the garbled message when other air raft tried to contact them