Windows has millions of users. If Microsoft went out and said: "Hey, this game you all like has a bug in 64-bit versions, here's the source, any ideas?", I bet tons of people would have invested a couple of days. As you see in the blog post, it was not even possible to invest that amount of time into the problem within Microsoft.
Open source is not magic in the least. It just enables other people to try and help out if your project means anything to them.
If the game had been open source from the beggining, it probably would not be an undocumented mess right now. Sure, open source is no guarentee for quality, but open source and popular means that many eyes will look at the code, and it will naturally develop documentation.
Open source is not magic in the least. It just enables other people to try and help out if your project means anything to them.