To express something in no uncertain terms means to express it very explicitly and emphatically, but in this context the speaker expressed themselves vaguely and without conviction
Consider the statement "I had expressed to him vaguely and without conviction that I doubted my ability to keep up with his style of life.". Based on this "I had expressed to him in no uncertain terms that I doubted my ability to keep up with his style of life." a true statement? Yes, no, or I don't know?