If the Synagogue of Libertines (Acts 6: 9) was Hellenist it would not have been regarded as properly Jewish by conservative Jews.
Then there arose certain of the synagogue, which is called the synagogue of the Libertines, and Cyrenians, and Alexandrians, and of them of Cilicia and of Asia, disputing with Stephen. (Acts 6:9, KJV)
Is this what Revelation 2:9 and Rev. 3: 9 talks about?
I know thy works, and tribulation, and poverty, (but thou art rich) and I know the blasphemy of them which say they are Jews, and are not, but are the synagogue of Satan. (Revelation 2:9, KJV)
Behold, I will make them of the synagogue of Satan, which say they are Jews, and are not, but do lie; behold, I will make them to come and worship before thy feet, and to know that I have loved thee. (Revelation 3:9, KJV)