A little bit of digging shows that the discoverer of the BadWinmail vulnerability is Haifei Li, who appears to be a researcher for the security / anti-virus company McAfee, judging by those blog posts, it looks like he specializes in Flash and Microsoft Office.
You can bet that a company like McAfee has special agreements with Microsoft which gives them some sort of access to the source code or special testing environments in exchange for telling Microsoft about any vulnerabilities before going public with them.
Also, most veteran security researchers are very good at reading raw assembly code, so having an .exe is just as good as having access to the original source code.