That is absolutely untrue in many (but not all) cases of "intellectual property". Let's take some extremely valuable "IP", Microsoft's "Word" word processor.
If one person had exclusive use of "Word", the "IP" would be worth a great deal less than it is right now, when almost everyone has "Word". The property here is called "network effects", and it also applies to physical property. If only two telephones exist, they're a good deal less valuable than if everyone has a telephone.
the value of a good that is being sold depends on the supply of that good in the marketplace. if there's no change in the supply in the marketplace (i.e. if the dude kept the sourcecode for personal reasons and never shared it), then there is simply no harm whatsoever.
