If you listen to the many taped conversations you will notice that it was Arcadi handling the day to day operation not the others.
How can they all be underbosses ? I personally think Sebastiano meant it in a different concept, maybe each having their separate roles. Some were sayng Renda handled the finance, Nick Sr. was a consiglieri, and Sollecito the underboss and Arcadi acting boss. But Nick Sr. being old school and judging his past behavior, I think he had Arcadi on a short leash.