Hope Zulip's discoverability improves.

In what sense?

I assume they mean the fact I myself know what Mattermost is but I've never heard of... now I even have to go back and load up the comment to find it's name again, Zulip

Exactly this. :)

Zulip is a kind of annoying name, and every time I encounter it it's in the context of some open source platform hiding their community discussion forum behind a login. I'm left with a not very great impression.

FYI, for a while now you can mark any Zulip channel as public, which means the chat history for that channel does not require a login to view. See https://zulip.com/help/public-access-option for more details.

I don't like the way they push you to give a subject to every discussion. In a way it's more like a newsgroup replacement than Slack, whereas Mattermost is a straight-up Slack clone.