As previously stated, this isn't listing the rooms or groups inside a slack account, it's listing the slack accounts that you might potentially be trying to login to.
IMO, this seems like more a security issue of the individual creating slack accounts for, a) naming the accounts for a specific (potentially revealing) sub-set of their company, and b) turning on the feature that allows anyone to create an account if their e-mail matches the domain.
The company I work for uses Slack but has this second feature turned disabled and our company is not listed when you try and sign in with a bogus e-mail account.
IMO, this seems like more a security issue of the individual creating slack accounts for, a) naming the accounts for a specific (potentially revealing) sub-set of their company, and b) turning on the feature that allows anyone to create an account if their e-mail matches the domain.
The company I work for uses Slack but has this second feature turned disabled and our company is not listed when you try and sign in with a bogus e-mail account.