«لِبَدَاهَتِهِ ... بِبَرَاعَتِهِ»
Because of his spontaneity ... because of his mastery.
The two terms name distinct qualities. Badāhah concerns how quickly an answer is given; barāʿah concerns its artistic quality. A quick reply can be poor, while a polished one may take time. Here the audience sees both.
Their parallel endings create a rhymed-prose effect. The sound-pattern does not erase the difference in meaning; it makes the sentence economical and memorable.