My take on this is that CR+ is not separate inventory to paid fares. I'd hazard a guess that there's some backend match happening so that there might be six seats available for the cheapest CR+ fare bucket and six seats available for the cheapest paid fare bucket, but only six seats available in total for those two fare buckets. Once six seats are exhausted (potentially a combination of CR+ and paid fares), both will go to the next fare bucket. I would also expect that the most expensive cash fare buckets wouldn't be matched to a CR+ fare bucket.Hence the concern and surprise about 20,000,000 seats open to CR
QF have probably priced this so they are indifferent between someone booking using cash and someone booking using CR+ points.
Hence 20m seats available for CR+ is very much possible.
As an aside, if this is how the mechanics work, have a think about what that does to the price of (paid) airfares...