While I do not know the exact language the captain used, but if he said only that the source of the smoke was external to the plane and not that the source was just the exhaust from a backfiring tug, could that not be interpreted by PAX as "there is a raging fire right outside the plane, but not to worry, it has not breached the fuselage." There are many examples of fires that are initially external to the fuselage quickly entering it. I would hesitate to say the PAX were idiots to have sought to evacuate.