Didn't believe? But that's true. Earlier I too didn't believe and thought it's related to April Fools’ Day. This is one of the most elaborate April Fools’ pranks I’ve ever seen and it’s a bit closer to the reality.
Microsoft says - The MS-DOS Mobile preview is an essential download. Whether you’re going back to BASIC, or simply booting into DOS for the first time, MS-DOS Mobile marks the next step in Microsoft’s reinvention of productivity.
This OS allows users to run numbers of pre-installed apps and that too also from the popular C:\ prompt. You can also check the demonstration video of the MSDos OS on Lumia Smartphones.