Earlier quoted context omitted.
MS-DOS is maybe the ugliest operating system I ever had the displeasure of using. There's very little to learn from operating system design point of view in there, and better sources are available under more permissive licenses -- ie open source licenses. It looks like a PR stunt to me.
Given Satya Nadella's CEOship, and the sort of guy he is, I'm fully ready to believe that this is the start of a wider code release. It'd make a bit of sense that they have to work through the licensing to release MS-DOS before they release the source to any products that use MS-DOS at the core of them--like, say, Windows 95.
I mean, Adobe also released the source code for an old version of Photoshop. So old that it didn't even have layers. It didn't signal anything about the release of source code for say, Flash Player.
And look at which version of Word they released -- 1.1. Not even 2.0, because 2.0 was the version where it became mainstream.
Microsoft has already released a lot of source code that's a lot more useful than DOS and WinWord. For example, the .NET Framework Reference Source.