The key to the CAO change is (as stated within the NPRM) that it will .....
Provide an avenue for an air operator to conduct operations using the number of cabin crew members used in an aircraft’s successful evacuation demonstration up to a ratio of 1 cabin crew member for every 50 passenger seats or part of that number.
If the Type Data Certificate does not include such a demonstration during the certification process then it won't be available on that aircraft type. If the demonstation has taken place (successfully) during type certification (proving that evacuation within 90 seconds using that ratio is possible), and the operator can show CASA that their emergency procedures will be able to achieve the same, then compliance with the new CAO would be achieved, as I understand it.
I'll be interested to see if CASA will still want a "safety case" and a partial evac demonstation. I'm guessing that with the new CAO published they would not, but we'll have to wait and see.
Given the rest of the world do the same (as waren9 points out) it will bring our Australian regulatory stance on this element in line with the rest of the world, which seems like common sense to me.