DOS wasn't a very complex OS and has already been reverse engineered more or less completely. Apps like DosBox already exist. It might cause a couple of minor revelations if/when the source is finally opened but I doubt it will have a big impact.
They trickle out versions over the years. This time they published version 4. The versions up to 6.22 (standalone) and up to 97-8 (part of Windows 9x-ME) are yet to be published.