I think this is where I will take it to mean that death by CO2 must be followed by a second type of euthanasia. So even if you use CO2 in a long prolonged exposure, you should still follow THAT by some other method to dispatch the snake "for sure". This could mean putting it into a freezer, but if the CO2 is not reliable enough to kill the snake, there would be a slight possibility of the snake coming to enough to be aware of freezing too?
To me, death by CO2 seems almost impossible in a home setting. Unless I suppose, you were to use a airtight container flooded with the CO2 and leave the snake in there for a extended period(hours?) and then you'd still want to do something more afterwards.