I'm not sure what you mean when you say: "....bought in full". It's my experience that most, if not all of the pre-construction developments in Toronto required purchasers to put a deposit of 15-25% toward the purchase price until the project is completed, with the transaction coming to an end only when Title is transferred. In light of such a process, it isn't possible to 'buy in full' as the property has yet to be completed.Sold out doesn't mean that people bought in full. In a development this big (1200+ units) I'm sure there will be more than one person who can't close on their unit. Nevertheless, good to hear the project is on schedule.
I think you are correct in saying that there will be some purchasers who can't close their units upon completion due to personal circumstance. However, AKS' comments above are correct in that the developer can seize the deposits, re-sell the unit, and/or sue the purchaser for the shortfall (if any).
With this particular project, I don't think the builder will have any difficulty re-selling a unit within days if they had too from a defaulted buyer.