Apologies, but since I do not coach anyone (nor do I have a coach), I’m not the best person to answer that question. Gerald or one of the other more knowledgeable members may have better info.
ETA: This ( Coach & athlete activity comments & notifications ) would be a great place to ask David for clarification if the athletes are not seeing alerts, since he clearly intended the comments to trigger notifications.
Also look here for more details: Comment Notifications