This is one brilliant idea. Containers are prone to rust if they are not well treated. One can pick up a good container in Cape Town for under R10k and bury it in your yard without anyone ever noticing. Another problem this specific guy might have is that the container does not look that strong from above, if a car drives over the container it might fall in. The best way to ensure a safe container underground is to put beams right across and leave some space between the beams and the container with pressure gauges, that way you can also track the pressure from above and track any changes.
This guy also used 6inches of cement, that might be a bit much but the cement would make it a lot stronger.