Well we already had a proven self-hosted Slack alternative called Mattermost that pretty much everyone enjoyed. The push for XMPP is because some of those fanatics live in a parallel world where everyone is using XMPP and they'd be able to talk to people on other servers with it. Of course reality is very different.

I'm leaving soon anyway so I wish them the best of luck with this (and I feel bad for some of the people who actually wanted to get work done instead of fighting with broken and outdated tools).