That was a good summary Sunfish.
I believe it is indeed true, if you are "denied" a CASA medical, (that is, CASA says NO.) you can't jump over to RAAus.
But, a lot know they will be denied a medical next time, (or they just don't want to go through all the hassle) so the solution is to jump before putting yourself through all the hassle and possible/probable denial. (If you don't apply for a medical, you can't be denied.)
The the aviators self-certify each and every day they fly, (am I feeling good? Yes!) and go on to fly safely for many more years. But in the VH world, flying the same aircraft in many cases, you are a dnager to humanity.
RAAus and the others have all proven without a shadow of a doubt over 30-40 years that self-certifying is safe - for private flying.