Of course for a product like Windows or Office, what you say definitely applies. I'm not sure I agree here though, for something like this I can quite easily imagine scenarios where one person would do a substantial portion of the work. The code samples provided look to me like there's not a lot of separation between the code and UI anyway.

Plenty of large companies I've worked for have had single people working on side projects that later get the attention of management and have more resources diverted to it. Look at Google and their 20% time philosophy for a well known example. Alternatively, imagine a situation where a senior developer is given a project and allocated a couple of junior developers to help. It wouldn't be at all difficult for the senior developer to pass off the stolen code as his or her own.

Of course we'll probably never know exactly what really happened, but if you're right then the rot is very deep indeed.

Of course it's not really Microsoft that copied it, it was someone within Microsoft, who clearly didn't think things through and is probably rather unlikely to be employed there much longer. Of course that doesn't mean it's not Microsoft's problem since they now have to do damage control due to the egg on their faces.

As an aside, I wonder how feasible it is to put some automated checks in place to compare a signature of some code against every other known piece of open source (or otherwise?) out there to search for similarities?

You either didn't RTFA or you're just trolling, because all these points are covered quite clearly.