You weren't misreading but are confused. You're talking about 'cash & avios', whereas this is 'miles & money'. The latter requires redemption availability, whereas the former does not, i.e. it's just a discount on a cash fare, and as such is limited to certain tickets where BA chooses to offer it. In this case, BA offers the ability to pay slightly fewer avios for a redemption in return for voluntarily paying additional 'charges'.