I doubt if Mandel would doubt the veracity of Forest's case, since he has over the years used many similar instances and arguments in attacking the Stalinist regime. In this case it seems that Forest was kicking an open door. It is implied that Mandel has attempted to suppress the facts that Forest brings forward. Such a case could only be made if she can prove that Mandel has consciously chosen to omit evidence that would run contrary to his own case. To accuse someone of dishonesty one needs chapter and verse evidence, not speculative accusations.
Forest fails to provide such evidence.
She is broadly correct in her resume of Stalin's foreign policy from the early 1930s as being an attempt to exploit interimperialist rivalries. But this does not thereby disprove Stalin's myopic conduct regarding Hitler's plans for attack. I found nothing in Mandel's account which would indicate anything else. Therefore, I am at a loss as to why he is accused of dishonesty. Forest seems able to paint history with a very wide brush stroke when it suits her.