MICROSOFT MUST be very desperate to appear as "open" as GNU/Linux/Android even though Windows is definitely not. Several journalists got bamboozled by Microsoft's latest PR charade, which involved exposing source code of legacy stuff that's of no use and nobody uses. Microsoft's thugs are once again interjecting themselves into museums (public space), just as Bill Gates did over the past decade or so (the Gates Foundation was paying establishments like these to glorify Gates and warp computer history, omitting all the crimes).
It is bad and could be very bad in various ways. It is historical revisionism and copyright propaganda. It may also be a trap for free software developers.
We can be sure that the source code is washed of sabotage for competitors. That would be revisionism. The Engadget article itself is either revisionism or ignorant - Gates simply purchased/licensed/stole QDoS, the Quick and Dirty Operating System to make MS DO.
From a copyright perspective, Microsoft is pretending binary code finally enriches the public domain but that's a farce. We can't verify that this is the source code they worked with, nor should we trust companies to finally come clean decades later. This is very important because copyright protection is only granted in the US if it advances the state of the art and public domain. None of that happens here. This will be used as propaganda the same way the Gates Foundation is - a germ of truth will be blown out of proportion to conceal an ugly reality.
Finally, Microsoft never really gives anything away -this code is poison and should be avoided by free software developers and competitors alike. Let's look at their "agreement". Oh yeah, you don't even have freedom zero because there are limits on personal use. Personal use if only for "non commercial purposes," it appears that even consulting based on results of tests are prohibited. You may not share your copy or your modified copies. It's like they looked at the four freedoms and negated each, and that's just the first of eight restrictions. One of the nastier restrictions limits damage to $5 for anything, including things Microsoft should have known about - like anti-competitive sabotage.
FU Microsoft, I'll stick with DosBox and other free software. You can keep your fake old crap and I still don't think you have legitimate grounds for copyright monopoly.
Microsoft actually bought the rights for QDOS (stands for “quick and dirty operating system) from Seattle Computer Products in 1981 for a paltry $25,000. What happened next is computer history.