Yes is would be dangerous if I used a closed container, without a temperature/pressure relief valve. Obviously, in both the videos, those key pieces of SAFETY equipment had been removed and the hole plugged.

If you have ever seen a properly equipped tank go into thermal runaway, the T/P valve lets loose well before an explosion can happen.

In this case, YES, I would plumb in a T/P valve. In addition, NO fire would be lit without water flowing through the heat exchanger, and would continue to run well after the fire was out. Under no circumstances would this be left unattended. There would be no pressure vessel (tank) involved in this build, if I do decide to go through with it.