Note 5 at Phil. 1:16: What is Paul referring to? How can a person preach Christ motivated by contention and insincerity? And how could that add affliction to the imprisonment that Paul was already suffering?

This is probably referring to people who had heard the claims Paul made about Jesus being raised from the dead but didn't believe him. As they repeated these details to others in mockery, the gospel of Jesus' death, burial, and resurrection was spreading and the Holy Spirit could use that. Regardless of the motives, the truth was spreading and Paul was rejoicing that people were being exposed to the claims of Christianity.