I don't think Ruiz is even world class. Let alone better than Zhang or Kabayel. He wasn't considered a top 10 heavyweight going into the fight. Povetkin was 39 and looked poor before prior to and after facing Joshua
Vlad was 41, Povetkin was 39...you see how you missed some years there? 283 pound Andy Ruiz is not significantly better than anyone, much less Zhang, Sanchez, Chisora. As many others have said, it's an overhyped, oversold resume.
We aren't comparing Ruiz to Kabeyal directly, here. We are comparing opponents. Zhang is a post-40, hit or miss guy.