In short, you won't be able to withdraw that bitcoin, you are just able to cash out the value that's gained, or lose the value in case it drops.
Been down this road/ discusion before with gold silver ETFs. That you can not actully book an appointment with the vault to cash your ar of gold and collect it or have it delivered
Again; Do you own a share of this and dose your ETF company own bitcoin that is fractionlized for you on your behalf? I mean did they initally go out and buy bitcoin them selfs?