Ah, that’s a bunch of crap!
"almost three feet underground."
That's pretty fer. I don't get it either, they didn't mention any injuries.
The tank has three feet of dirt on top of it. The septic tank itself is probably five to six feet tall, plus the three feet that the tank was buried underground, so she was standing at the bottom of a eight to nine feet hole in the ground. The water and sludge probably made for a soft landing.
I suspect the septic tank was either recently pumped out or it was an abandoned tank. If it was anywhere near fully operational, she would have drown.