It's a dark pattern. They pop up a dialog with just an email field and a [Next] button. Makes you think you have to enter something before you proceed. But you can leave it blank and move to the username/password stage.
Notice all the sibling comments that insist the email is required now? It’s an understandable misread of that sign up flow. Only one field and then a Next button.
Almost no other input flow allows you to hit “next” when the only form field hasn’t been addressed yet. Why not put username, password, and email on one page instead of splitting it up the way they do? Then you can mark the two fields as required and the third as optional.