Were the Jewish believers Paul persecuted still under the Law?
Yes. Paul himself recounts persecuting those Jewish believers, and they were still Jews under the Law. This shows that Jewish believers at that point had not been removed from the Law, which is the key point being emphasized. That is what the text underscores.
Les explains that Paul recounted in his own experience how he persecuted those Jewish believers, but they were still Jews under the Law, and that is what he wants his audience to see. Les also notes that when chapter 10 is reached, Peter is still a law keeper, though he says that is for another time. The same passage then marks the big turning point in Acts: Saul of Tarsus is a chosen vessel to bear God's name before Gentiles, and also to kings and the children of Israel. So the persecuted Jewish believers were still functioning under the Law before this wider Gentile outreach.