The Pharisees were a Jewish sect that opposed the message of the Gospels. Yet, the book of Acts (also written by Luke) reports that many of them became believers in the Gospel. There are two good reasons this would not be a fabrication.
(1) There would be zero motivation to portray Pharisees as becoming followers of Jesus.
(2) If this were a lie, it could be easily exposed by the other Pharisees who were actively working to subvert the efforts of the disciples. No one would invent details such as these.