Here Paul describes a situation that could be true but is not. Here "flesh" is a metonym for anything concerning one's body, such as one's ancestry, whether or not one has been circumcised, and what one does. To have confidence in the flesh means to believe that those things can make a person right with God. See how you translated a similar phrase in 3:3. Alternate translation: "However, I could trust in something about myself to make me right with God. If anyone thinks he can trust in anything about himself, I could trust in myself even more" (See: [[rc://en/ta/man/translate/figs-metonymy]])