From my understanding, once Montreal approve the case transfer, they will request the foreign consulate to send the case file to Montreal. Once the file is received and based on their work load, Montreal will schedule the interview. Your parent's case file might have been transferred to Montreal already and waiting for interview scheduling. This is my guess. Also, I'm not sure if the consulate to consulate transfer will change the status in NVC.
My case transfer request sent to Montreal in July is still pending and no response yet other than one response in September that they are reviewing the request. Did Montreal send you an email accepting the case transfer in July?