How could the Helios 522 passengers have survived so long without oxygen? I've noticed some strange inconsistencies regarding the documentation of Helios Airways Flight 522.

So first:


  Autopsies on the crash victims showed that all were alive at the time of impact


But then:


  The emergency oxygen supply in the passenger cabin of this model of Boeing 737 is provided by chemical generators that provide enough oxygen, through breathing masks, to sustain consciousness for about 12 minutes


So, the oxygen masks in the passenger cabin automatically deployed at 18,000 feet, probably around 09:20. The aircraft did not impact until 12:04. 2 hours, 44 minutes later. Obviously the oxygen masks didn't keep them alive for that long, so:

How could they have survived so long?
To add to Daniele's answer, from the final report:


  The  forensic  report  concluded  that  the  aircraft  occupants  had  heart  function  during  the  impact.    The  report  noted  that  this
    did  not  necessarily  imply  that  they  were alert.    The  report  further  estimated  that  they  were  in  deep  non-reversible  coma  due  to 
  their prolonged exposure (over 2.5 h) to the high hypoxic environment. 


So, again, saying that they were alive does not mean that they were well.