I lead the Zulip project, and we would definitely have been excited to do the work to make Zulip available. Unfortunately, they didn't contact us.

After seeing this blog post, I emailed the OP yesterday offering to add whatever they need to evaluate Zulip (which sounds like is just SAML authentication work that's was already on our near-term roadmap). He replied that they're too late in their evaluation process to consider Zulip :(. I really wish they'd thought to get in touch with us about this concern.

Zulip is open source, and the community is both large and very active. So at least they can open a github issue explaining the problem. And if they want to use the tool, they can just submit PRs like everybody else.

Even if it's just for a PoC with a 1/5th chance of being actually selected in the end?