Indeed, Federation was only a part of their Enterprise offering. :rimshot: It is only Slack-to-Slack, so it is not interoperable in that way. Shared channels are a thing now too, and danged if I can keep up with all the new bells and whistles.
Slack is very much an organizational groupware product, with chat done well. Not the utopia federated XMPP promises, but frankly, XML just needs to die. (Which, not coincidentally, is part of what attracted me to work on the telehash specification.)