I'm disappointed and growing hopeless about the state of software engineering at these companies that this sort of issue is not caught in engineering design documents, during development/debugging, or during code reviews. Any competent engineer should have the sensibility to have seen that the implementation they've designed or programmed leaks private information in some scenarios.

Or, perhaps the UX design team intentionally decided that mentioning the Twitter username associated with the email address would be a "helpful" piece of info to present at this point in the login/signup flow. In this case, too, the design team should have known that privacy far outweighs any potential helpfulness.