If the other "end" you are talking to is their server, then it's end-to-end encryption. If the other end is another person's computer, and their server is only

If the other "end" you are talking to is their server, then it's end-to-end encryption. If the other end is another person's computer, and their server is only one jump on the way there, then it's not end-to-end. Clearly, with Slack, "their server" was not the other "end" in anyone's mind; so they were being disingenuous. But for many cases (banking), their server _is_ the other end.