I think the intent was to say that if the professor had a reasonable belief that the test bank was secured and not accessible to students, then they probably should have had that same belief, and not used it as a source.
Since the professor stated on the first day of class that he would be creating his own tests, there was no reason for the students to even consider the possibility that a test created by the textbook publisher would be used as the official test.
I don't necessarily agree with that.