These panels prove it's all due to Saul if anything. Kuzan wasn't sure of his decision but he spared Robin anyways because Saul wanted it, so he wanted Robin to show him whether his decision, which is Saul's decision by extension, to spare her is the right decision. The Ohara is alive dialogue refers to Robin because Ohara won't die as long as Robin lives.
Simply put, I think Kuzan doesn't know Saul survived Ohara. Kuzan decision to not make sure Saul is dead through and through did definitely help him survive the attack though. I think Kuzan probably did it because he couldn't bring himself to finish him off, not because he wanted to let him escape. Kuzan's relationship with Robin is completely built upon Saul's death, period.