The difference is it’s harder for them to cancel after delivery than it is for them to just not start the order/accept the offer in the first place. If they try to cancel you can decline specifying a reason if you want to (though they can request cancellation again etc.). If they want to cancel through CS they have to convince CS to cancel/that there’s a valid reason for doing so (eg. if the seller hadn’t given them what they asked for).
If you wanted you could have shown some sort of demo that showed you could do what they require (if it doesn’t take long) but without giving them everything (ie. without the source code if it’s an app and they want the source code too). Maybe just something that proves you can do it but without giving them it in a usable form.
But like has been said, it’s best not to do the full/proper work on it without an order.