The tanker could have been filled with a gaseous substance at a higher temperature than the surroundings, and this demonstration may have taken place at a location that was at a temperature below the vaporization point of said gas. Therefore, as the gas inside this tanker cooled to the temperature of the surroundings, and if the tanker was well sealed, the gas inside would begin to condense and form a liquid, which would cause about a 1000x or more decrease in volume. However, since this is a constant volume system with a constant amount of material inside, this would cause an equivalent decrease in pressure inside. This huge pressure decrease may be sufficient enough to cause an implosion.

You can do this with a plastic bottle; put some really hot water, just a tiny bit, enough to get steam. Cap it, then cool it real fast with water.