It can, if everyone jumps in for a swim at the same time when becalmed -- with sails up. Even a light breeze can push the boat along faster than most folks can swim for any great distance.
Of course, the other possibility is alien abduction.
What if they were hove-to; two went in for a dip and the 3'rd decided to take a swim but forgot it would leave no-one aboard? The boat could still get away if a wind shift caused it to set sail on the opposite tack.
But engines running while the sails were up is another cause for concern, were they in neutral for charging or were they driving the boat along like they were motor-sailing?