The two descriptions are not a genuine contradiction, because they concern different relationships: gentleness toward believers and firmness toward disbelievers. Humility in one setting does not entail the absence of courage or dignity in another.
Taken in isolation, «أَذِلَّةٍ» might be misunderstood as general humiliation or weakness. «أَعِزَّةٍ» balances that reading and makes the intended moral distinction clear. The rhetorical value of takmīl is not simply the occurrence of opposites but the removal of an inappropriate inference.