Which is why these installs usually involve all sorts of external waterproofing, concrete encasement, galvanic/cathodic protections & sacrificial anodes, drainage systems, and interior climate control. They don't just bury the expensive fuselage and call it a day.
The video says the guys wants to put the hull under a 3 inch fiber-reinforced concrete vault, which would be supported by the plane. The guy's word: "I'm hoping the plane can take that sort of weight". Then another thicker layer of concrete this time reinforced with mesh.
I for sure hope the guy hires good engineers to do the actual calculation and the whole "I don't know what I'm doing" is just for the show.
Serious question - is burying a plane fuselage to create an underground bunker a common enough thing that there is a usual way of doing it?