It didn't come as a surprise; he already knew what had happened, after, thanks to Rukia. That didn't mean that he understood it. And maybe he didn't want to; it was easier to understand his captain as he knew him. He didn't know how to interpret what Rukia had said, other thinking that Rukia would do whatever bizarre mental gymnastics were necessary to look favorably at her brother. And he understood the desire, he really did.

...it just wasn't something he could do. He had to rely on what he knew.

This question, at least it was easy to answer honestly. "I don't know, sir," he said. And it was true; he didn't. He couldn't even try to put himself in Byakuya's place and guess. Renji had never been that much of a stuck-up asshole.