Not at all: Paul's strong response does not contradict his earlier statement in 3:1–2 that Jews have an advantage. However, this advantage has not benefited them because they did not obey God's commands and encountered God's punishment. Jews, like Gentiles (here, Greeks), have sinned against the revelation of God and stand condemned.
under sin: Being "under" something means it controls a person. The main problem for humans is not only that they have sin, but they are slaves of sin. The solution to this problem is the freedom that Christ Jesus provides. He frees people from both the penalty and the power of sin.