That's not really a problem though. Everyone uses 20 different communications platforms. SMS, Hangouts, Hipchat, Slack, Facebook, Twitter, Snapchat, Whatsapp, etc all have different logins.
It may be nice to combine them in one place but it won't prevent anyone from using them.
The problem is, there is no standard for truly owned identities besides usernames and passwords, and those are frowned upon as "different accounts". And many services seem to not want any interoperability at authentication layer and to accept third party (esp. competitors') assertions as credentials.
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.)
It may be nice to combine them in one place but it won't prevent anyone from using them.