My understanding is that Zulip was called Chime internally.
Management pushed teams to use the tool, and it was tried but the interface was not polished.
Slack was already out and despite how it is seen today, slack far more appealing.
Slack’s interface, single sign-on and mobile app availability made it hard for Chime to compete.
I don't think this is accurate. At least, I'm aware of a chat project called Chime at Dropbox circa 2015 that was not based on Zulip and at least at the time did not materially involve anyone from the original Zulip team.
(I suppose some of those things could have changed since I last heard anything on the topic).
It seems additionally strange that after buying Zulip in March 2014 a wholly separate chat application would be in use. But stranger things happen.