We do not know this coroner's medical sophistication. Also, at that time Missouri was using Leuchter's "execution machine" so the executioner only had to turn a key to set up an automated sequence. Two executioner's in a different room, had no view of the inmate. While most states either did not buy a Leuchter machine, others were quick to discard it after a few glitches like what happened to John Gacy.
It does not take a brain surgeon or a forensic pathologist to determine that a restraining strap was too tight, but it might be too much to expect from a DOC employee or other Neanderthal.