Reading Receipts needs no account. Publishing a prediction, or agreeing or disagreeing with one, does, so that every entry has a real owner who cannot be impersonated and nobody can be counted twice.
Your email address is never published, and nothing is copied from Google. If you continue with X, your X username and picture fill in your public identity on Receipts wherever you have not chosen your own; a username or thumbnail you chose is kept. Receipts shows your saved username, or the stable User_ number you earn by publishing if you have none.