I kinda hate that Microsoft gets to take credit for being magnanimous with yet another product they never created.

The TL;DR: The Zorks were created by several guys at MIT who later formed Infocom. Infocom eventually sold to Activision, Microsoft bought Activision and voila--"Microsoft is open sourcing Zork".

They're not taking credit for the product; they're taking credit for _open-sourcing_ it. Which they did.