This article/video really rubs me the wrong way. These strawmen who are being torn down for the most part aren't building "tunnels". They're building glorified 8-10ft foundations and basements with dirt over the top instead of structures, 1970s hippie "underground homes" basically. They're calling them tunnels and bunkers for clicks and views.
To then take that naming at face value and pontificate about code and engineering is very much a two slights of hand not making a right situation. Furthermore, a civil engineer doing so is deep into "man won't understand what his salary depends on him not understanding" territory.
I know that the many HNers from the seismically active portions of the US will have no frame of reference for this but there are portions of the world where for hundreds, sometimes thousands, of years basements were built with less than scant engineering. The sort of "just barely below dirt" construction most of these amateurs are engaging in is on that order of complexity. Based on my observations via Youtube, these amateurs should be more scared of their own temporary construction rigging and material handling solutions than the forces their structures must hold back.
The primary practical engineering challenge and hazard these structures face is that there's nothing stopping someone from driving a point load of undefined size over the top and that has serious implications for roof strength.