What prevents you from using a same identity to log in to multiple self-hosted instances?
I think it's the same. As far as I get it - haven't used Slack much - with Slack you either have to use different email address to create account for each team (subdomain), or you have to be explicitly invited. Or maybe I got a wrong idea. Either way, while I'm not sure all self-hosted alternatives have options to consume external identities (be it third-party leased ones like Google account or self-originating ones), at least some have those. Which means the authentication is fairly transparent.
It may be nice to combine them in one place but it won't prevent anyone from using them.
This applies to Slack too - to best of my knowledge, one can't log in there, using, say, GitHub identity assertion as a credential.
But I'm not sure how Slack has anything to do with this. You're right. They're just a yet another platform with its own non-interoperable account system. Don't see how it's better than anything other, except when everyone is already there, which I find hard to believe. (But any other platform that knows how to consume external credentials is wins in this regard.)
