He was working on the assumption that there was significant pressure built up in the thermos. Puncturing it would release that pressure causing a potentially dangerous explosion. The same premise as puncturing a balloon except the balloon in this case is made of metal and glass and the amount of pressure is significantly higher.
If you're lucky then the top of the thermos is plastic.
Safe way to solve the situation with a metal thermos with a plastic cap: clamp thermos to workbench, pre-drill hole in large sheet of plywood, drill through pre-drilled hole into thermos. Depending on how complete the sublimation is, there may be a boom and you, the plywood, and your drill may potentially get kicked back a meter or two in the worst case, but your plywood shield would likely spare you from any high-velocity plastic fragments that would cause you injury. If you're lucky the pressure is still low enough that it just blows plastic out of the spiral of the drill bit and there is little other than the plastic shavings to work around.
You would definitely want to be wearing earmuffs though.
Funny. Being from a very rural area my solution would of been to walk outside and hoof the bloody thing as far as I could into the surrounding fields and have a cup of tae while waiting for the boom.
