Anyway there is no possible argument that the meanness of those replies is productive; it would clearly be more useful and direct to omit it and speak clearly about the problems instead of implying the person is an idiot for not understanding their mistakes.
Moreover, why are you "surprised"? Do you live in a some toxic alternate reality where people talk like that regularly? For most people these messages are the only time they'll ever see someone being that mean in a professional context in their lives.