Currently when a draft order is closed it creates a new order from the draft order, this new order creation is what triggers the customer order confirmation email to be sent, and this is not something that can be disabled at this time.
However there is a workaround to prevent this from being sent, by not including the customer details in the draft order before it’s closed. Then after closing the draft order, you can add the customer details to the newly created order with an orderUpdate mutation.
I confirmed this on my own test store with draftOrderCreate, passing the shipping address and billing address, but leaving out the email, phoneNumber, and purchasingEntity fields from the mutation input. This creates the draft order without a connected customer profile, and when the draft order is closed there is no order confirmation sent.