I think someone as adept in the spook world as Snowden is would likely have a hidden stash somewhere. But that doesn't matter wrt this argument.
What matters here is that there is a very real and significant possibility that he could do something more. As long as that possibility exists, he can not be granted a blanket pardon. He could certainly be pardoned for specific offenses for past activity, but first those need to be specified by indictment, which has yet to be done.
We could argue over whether the possibility is 10% or 1% or 0.01%, but that would be foolish. You cannot completely argue away the possibility, and there can be no blanket pardon under these conditions.
Maybe a car analogy is necessary here. Obama, and every sitting President, has the power to pardon Johnny Queue Publique at any time after JQP has been accused of stealing a car. But there is no provision that would allow JQP to be pardoned for a car theft that has not yet occurred. The President cannot give anyone a free pass to steal a car tomorrow. Not even President Trump will be allowed to do that, any tweets to the contrary not withstanding.