A propane cylinder will hold quite a bit of pressure, if you get it purged correctly before using it. Someone will get the purged tank mixed up with the real tank of course, human nature.
The sad thing is that there are some people who will see that video and go out in their back yard and start heating their real propane tank, which will of course explode and leave a very large crater about the size of a house.
A very poor choice for a boiler.
If you drill into a proane tank without purging it completely, it will explode, even if it is empty.