This literally doesn't make any sense. From Halo 1 all the way up until Halo Reach Bungie was making Halo games, and it was Microsoft who decided that they should be exclusive to their home console. It wasn't up to Bungie as to whether their games came to PC or not, it was Microsoft's decision. With Destiny its more likely that Bungie were waiting to see if there was a demand for their game on the PC platform, and now that there is they will most likely be bringing it to that platform.