I was looking this up as well. When these batteries enter thermal runaway, the chemistry of the battery is basically self propagating heat which allows it to continue to heat up and ignite. It doesn't need oxygen to continue this process.
Burying it insulates it, which will increase the runaway, but isolate the issue. It's good for small batteries like cellphones and laptops because it allows you to move the battery somewhere else and contain the fire, but doesn't do anything to retard the runaway.
Water can cool it off and slow down/potentially halt the runaway. Someone smarter than me would need to comment on what to do after it's halted, because I assume it's still a problem waiting to reignite at that point.
Edit: This actually is a potentially interesting idea, a tomb vehicle that basically encases the burning vehicle in a heavily insulated trailer for relocation.
Not my area of expertise, but I remember in chemistry class we had a literal bucket of sand for certain types of chemical situations.