There are different kinds of training - it sounds like you got a good one, but it's not fully simulating a blown-door situation. There's a difference between less oxygen @12,000ft and the actual low pressure of 33,000ft: The higher altitude has about 1/3rd the pressure of the lower altitude, so it actively removes oxygen from your body (lungs, blood) faster. But simulating with an abrupt 33,000 feet would cause ear pressure problems, and it's a lot safer (and quickly reversible) to just play with the oxygen concentration.

Also, in this thread, I think you all know this, but wanted to make sure - a chemical oxygen generator "burns" to produce oxygen. The exothermic reaction is started with a small explosive; burning means its producing oxygen. Of course, in the 592 crash, they weren't properly packed with heat shielding, so catching other things on fire (plus the excess oxygen) was the cause of the crash. https://en.wikipedia.org/wiki/Chemical_oxygen_generator