I would suggest that the amount of oxygen at nose level depends on how deep the tank is, how far below surface the top of the tank is, how tall the person is and perhaps whether it was windy or not (and whether it was protected from winds by winds, trees etc)... there are lots of variables but most tanks are either 6 or 8 feet deep so she obviously was close enough to the hole where the lid normally goes that she got enough. The article also says that "...she had fallen through about a 2-foot by 2-foot hole in kind of the rusted metal top of the tank." Since she broke through, the hole was likely much larger than the standard 18" x 18" hole so even more oxygen came in. What I want to know is who on earth would have used a 'metal top'? Did they not know they were creating a death trap?
Metal septic tanks were common until sometime in the 1970s.The plumbers buried a metal septic tank outside my childhood home in 1960 as part of installing indoor toilet.Sometime in the late 1970s a hole appeared in the side yard. The tank rusted through and the sod fell in .Easily could have been a tragedy. The new one was concrete.
Steel tanks were lighter, cheaper and easier to handle but they polluted a lot of places with sewerage or gasoline.(The two most common uses of buried tanks.)