Well there's two arguments to be made. They 100% gave him the source code in a grab bag of goodies. That's a pretty simple case of he has a right to the disc itself, so he could have just kept it (or resold it) and not published. Them giving him "stuff" was them "buying" the item back, not just them being nice (as you put it).

There's also an argument to be made that the code itself does not infringe on their IP, as this was the lost source code from the old edition of StarCraft (from how I'm reading it in the news). Losing this code specifically made Blizzard restart the project, so it's not even the same project nor a commercially released product.

The former argument is pretty black and white. The latter very tenuous.