This is not necessarily incorrect. Malarial resistance is a benefit to you, but a mutation that confers it need not be beneficial just because resistance is one of its effects.
For example, a mutation that made you immune to a dozen diseases, but also sterile, would reduce your fitness to 0. Game over, man! Game over!
In evolutionary terms, it is reproductive success that counts, not how healthy or long a life you lead. That a mutation confers a specific health benefit does not make the mutation beneficial; for that the mutation must increase your reproductive success, i.e. the benefits must outweight any costs.
So while the resistance is beneficial, the mutation that caused it need not be. Not if it has other, less beneign, effects.