You have provided good advice. I would add that people need to do a lot of research to find someone who is not a fly by night scam artist when finding a contractor that does this type of work.
I think that your first sentence could be easily misconstrued. While it may be appropriate depending on location and soil conditions to have the floor of your shelter 20 feet down... in most cases it would add a massive amount of unnecessary expense and complications to try and get the top of your shelter 20 feet down... for very little gain.
There are many books and articles available on this subject. Unfortunately a lot of them were written by people with less than stellar credentials.
Current California Law requires minimal resistance plus defensible space. Any insurance carrier with half a brain should be mandating concrete and steel with shutters for new construction in the wildland urban interface.
I’m a semi-retired Fire Marshal, and I’ve seen what works and what doesn’t. What is really needed is a defensible space, a decent source of water like a swimming pool with pump to supply water to cool critical areas of the structure, and fire proof construction including metal window shutters. Your basement then becomes your fire shelter, but only as a last resort if there’s not enough warning time to evacuate.
Advising people to stay in other than dire situations with no warning is a terrible idea, no matter how good their shelter is. Shelters can easily become traps if the entry points get damaged or covered over by collapsed structures, debris, abandoned vehicles, or emergency responders who don’t know they are there. And when you emerge from your shelter, you are stepping into a post apocalypse world. Better to have a good bug out plan, get out of Dodge early, and hope your structure doesn’t get too badly compromised by those who stayed and become desperate for any kind of shelter.