The narrators don't get paid by the retail price (and the contract doesn't suggest they would). Basically Audible splits the money spent on credits among the books that the credits were spent on in proportion to the number of credits spent on that book, but weighted by the retail price. I.e. a credit spent on a long expensive book pays the narrator more than the same credit spent on a medium length book, even though the credit cost the same to customer.
There's an undocumented revenue floor for price tranche of books. If the revenues per copy would end up lower than the floor in a billing period due to e.g. some promotion that flooded the economy with cheap credits, Audible makes up the difference and pays the narrator at least that minimum amount per copy. Apparently that floor is being hit every time, i.e. Audible is paying them more than they are actually required by the contract. The author thinks this is a smoking gun that proves how they're being cheated, which is an odd take.
Since the credits are far cheaper than the retail prices, it should hardly be surprising that the effective sales price is cheaper than the retail price.