Yes, it is really cheating. A publisher's test bank is very clearly not made for students to study from. The publishers warn students not to read them and try to make them inaccessible. In my experience, the professor almost always tells students what they can use to study from. Whether students can use past exams is almost always clarified - I wonder if a student asked in this case.
However, here the instructor has no way of knowing who cheated. He clearly made a mistake, knowing that using a test bank would make it easy to cheat. So, he should either let all the grades stand or he should point out the statistical anomaly and just make everyone take a new exam, which he did. I can't possibly see how students who didn't confess will be found out, especially if the exam was multiple choice!
However, here the instructor has no way of knowing who cheated. He clearly made a mistake, knowing that using a test bank would make it easy to cheat. So, he should either let all the grades stand or he should point out the statistical anomaly and just make everyone take a new exam, which he did. I can't possibly see how students who didn't confess will be found out, especially if the exam was multiple choice!