Let's also keep something clear. They're not making Greyhawk the default setting for the PHB. They're using it as a sample setting in the DMG to illustrate world building for DMs. The two are not the same.
In the article we got this from, it talks about how the PHB having a setting appendix that covers all the major D&D official settings. And that makes sense, because the currency of an IP is recognizable characters and places, and WotC doesn't want to discard anything valuable. So the PHB is likely to be full of art and references for all the various settings, not just one implied default one.