We need to discuss how Bungie has God tier lawyers.
Negotiated a deal to buy themselves back from Microsoft, and didn't get their talent roster completely gutted, leaving Microsoft to build a new studio to work on Halo going forward.
Left a teaser for Destiny in their last Halo project.
Above point means they may have started early work on Destiny while at minimum getting paid by Microsoft for their final Halo projects.
Got out of the Activision deal early, AND kept the Destiny IP/publishing rights
Secured funding for, and began early development for their next non Activision project while still working under the Activision deal.
It's not often you see Activision essentially get finessed with no getback.