Vern Unsworth, who's part of a large international team of specialist cave divers who succeeding in bringing the boys out using stretchers and a system of pulleys, told CNN that Musk's plan had no chance of working.
His assessment of the plan was brutal. When asked for his thoughts on the idea, he said: "He can stick his submarine where it hurts.
"It just had absolutely no chance of working. He had no conception of what the cave passage was like. The submarine, I believe, was about 5-foot-6 long, rigid, so it wouldn't have gone round corners or round any obstacles.
"It wouldn't have made the first 50 metres into the cave from the dive start point. It was just a PR stunt."
CNN's interviewer pointed out that Musk went to the cave personally to see the conditions. But Unsworth replied: "And he was asked to leave very quickly. And so he should have been."
Musk tweeted footage of a team practising using the submersible after the complex cave rescue mission was already underway. The idea was for a single boy to be put inside the chamber and taken to safety.
Musk went so far as turning up to the cave in person to leave a submarine there in case it was needed.
After the man leading the operation said Musk's plan "doesn't fit with our mission to go in the cave", Musk hit out at media coverage questioning the scheme and released correspondence between him and Richard Stanton, one of the British divers engaged in the rescue. Stanton had written that it was "absolutely worth continuing with the development of this system".
Musk's team has been contacted for comment.