In the end it is not about Paul or about any of the other apostles. It is about the gospel message that they preach. Paul reminds the Corinthians that the same gospel message was preached by all the apostles, and that they themselves believed it (1 Corinthians 15:1). This reminder serves as a foundation for the admonition that follows.
11 Whether then it was I or they, so we preach and so you believed.