I’d buy the fixed cost argument if the premium % decreased as the instance size increased. It doesn’t.
In order to show that the pricing has doubled, you'd need to show an increase in actual monetary cost to a customer, not that the margins have changed. The table of prices has no historical data, making change impossible to determine.
I'm sympathetic to the idea that RDS is overpriced, but can't say I'm convinced by this article alone.
If a food delivery company doubles their fees from 10% to 20% and the restaurant cuts its price proportionally, do you really say the delivery price is the same?
The total cost remains the same but the value the customer receives from the delivery service per dollar spent has been cut in half.