I can confirm that you had to enter the competition (registered, doesnt matter if you submitted a game or not) to be able to vote on the games, however outsiders can vote on the game's page that isn't connected to the game jam (for each game there is a page of the game and one of the game within the game jam group). But yes I do agree with the concern of people spam making accounts that give their game full 5 star ratings, but the staff that pick the winner should hopefully be able to easily tell any outliers because of that.