Correct: the right to distribute a game with that branding and those mechanics is valuable if you have the means (such as any copy of the source) to do so.

The source by itself is hardly (monetarily) valuable, pretty much to anybody. Blizzard clearly still has other copies, and anybody else wouldn't be able to do much of anything commercially with the code.

It's quite possible somebody has even already reverse engineered much of it, rendering it even less valuable by itself, even to copy-cats.