Maybe this IS a feature.
I was discussing Slack’s UX with a friend of mine (Gold&Bold) last week when he pointed out this exact thing.
I think this might be intentional. If you try to put yourself in Slack’s Product Team shoes and think beyond work teams, it makes some sense to keep identities separated.
You probably don’t want to use your real name when your joining the “My Boss Is Horrible, Let Me Tell You What He Did” community on slack.
And I do think Slack aims to serve more than just work teams. Even if they didn’t aimed at it from the start, it’s already happening and they’re aware of it.
This doesn’t mean the account creation process couldn’t be simplified by, for example, adding a “Use my default identity/Customize my identity” step but I feel it makes sense to allow customization.