The employment release agreement and that confidentiality clause binds the parties who sign the agreement. It also depends on the wording of the clause and the circumstances - so read it carefully. For example, if the witness is an employee of your old employer and your old employer must ensure that employees abide by the confidentiality clause, then your old employer has an obligation to ensure the witness keeps the information confidential. (Or vice versa if the witness was someone you had in for your benefit).