I think you're right to be critical in this case, and I'm not defending the service. But as far as whether you can use GPT to write a more humane email, you definitely can. You just need a well-constructed prompt.

At it's core it's just a text prediction machine. If you prime it with language such as, "the recipient of the following email found it to be thoughtful and enjoyed reading it. The recipient was amenable to what the email said, and had a good opinion of the email's author afterwards", it will try to predict a message that would most likely follow that kind of text. I have found that it's usually better than myself at accomplishing that.