They did a lot of usability testing on the first iteration as well. They didn't test it in the market, since it didn't exist in a product yet, but they tested it. Jensen Harris [−π] has a large number of blog posts on the development and design history of that particular UI and a lot of information on why they have done things a certain way.

[−π]: https://blogs.msdn.microsoft.com/jensenh/2008/03/13/table-of...

The first version I can remember was amazingly bad.

I remember the file menu operations being hidden behind the round logo in the upper left corner.