Before we get too carried away here, remember that whatever is happening behind the scenes could be derailed before Windows 10 support actually makes it to production hardware. These include both the WHCK (Windows Hardware Certification Kit) and HLK (Windows Hardware Lab Kit) which are for ensuring driver and other device compatibility with Windows. It remains to be seen as to whether Google will decide to allow its Pixelbook owners to install and boot into Windows, whenever they need to, to tap into Microsoft's rich software library.
As per the XDA Developers report, Chrome OS developers have already been working on booting into Windows 10, but the new mentions of certification affirm that it is now more official. There's also a question of, if it comes with Windows 10 preinstalled, if it would be Windows 10 Home, Pro or even in S Mode, which mirrors Chrome OS's lightweight sensibilities.
Changes to the campfire branch have also been slowly making their way upstream to the main firmware branch, but we don't know when, if at all, it will hit a stable release.