Hope coins.game can do something better for the drawing especially when it comes to a big reward like the iPhone.
Although it can be said as a free promotion where participants do not need to spend money to participate, but make it more transparent and fair will always be a better thing.
I agree with you that the contest must be as most transparent as possible but do you have any better idea than a raffle to pick winners?
None of contest participants can manipulate Bitcoin block hash to win a raffle and it is secured enough to say no manipulation can be used with on chain raffle. The rest is some criteria in the contest like how many applications one participant is allowed to make and rules like "First comes, first serves", "No duplicate entries will be accepted"...