The link you included points to a page Google Apps Scripts, but it won't help you. It says that the method "Gets the email address of the person who submitted this response, if the Form.setCollectEmail(collect) setting is enabled."
If setCollectEmail() is enabled on the form, the respondent's email would appear in the spreadsheet of responses (and the form would not be anonymous).
If the form is not collecting email addresses, there is no way to identify the respondent. In this case, anonymous means anonymous - not just hidden.