From BEA Interim report #2
The compression fractures of the spinal column associated with the fractures of the pelvis, observed on passengers seated throughout the cabin, are compatible with the effect, on a seated person, of high acceleration whose component in the axis of the spinal column is oriented upwards through the pelvis.
Fractures of the pelvis combined with compression fractures of the spine point to much higher levels of vertical acceleration than experienced in an ejection seat. Yes, airline seating is sub-optimum to support the human body and probably contributed to pelvic fractures. There are other injuries one would expect to see associated with very high g levels. See this student research paper for a general idea of the g levels associated with lethal levels of acceleration:
Acceleration That Would Kill a Human
In actuality, fatal human g levels in the vertical axis are lower than with fore and aft acceleration for reasons related to the content of the next to last sentence.