Aye so long as the actual supply of btc outweighs the amount of bitcoin you hope to withdraw, all is well. The iou's traded on exchange are inconsequential since they're within their own closed ecosystem. As soon as you wish to remove them and do anything other than simply passing ious back and forth, you have to pay the piper. And the piper only accepts real hard money.
As soon as you try to redeem 1 iou more than exists btc, then you're shit out of luck. So from a numbers perspective, sure there's more "bitcoin". But thats a meaningless number because as soon as you want to do anything with it, you have to exit to actual bitcoin. In reality when it comes to actual available liquidity, 21m=21m no matter how you slice it.