I asked Microsoft for further clarification and received the following statement from a spokesperson:
"Windows 10 is off to the hottest start in history with over 350m monthly active devices, with record customer satisfaction and engagement. We're pleased with our progress to date, but due to the focusing of our phone hardware business, it will take longer than FY18 for us to reach our goal of 1 billion monthly active devices. In the year ahead, we are excited about usage growth coming from commercial deployments and new devices -- and increasing customer delight with Windows."
Microsoft Windows and Devices chief Terry Myerson made the original claim at Build 2015, noting the 1 billion would encompass all kinds of devices that would run Windows 10 in some variant, including desktops, PCs, laptops, tablets, Windows Phones, Xbox One gaming consoles, Surface Hub conferencing systems, HoloLens augmented reality glasses, and various Internet of Things (IoT) devices. Officials said at that time the majority of those 1 billion devices would be PCs and tablets.
But Windows Phones running Windows 10 Mobile were also expected to help Microsoft reach that total by mid-2018. Since April 2015, the bottom has fallen out of the Windows Phone market, with Microsoft officials conceding that Windows Phone isn't much of a focus for Microsoft in calendar 2016.
Microsoft has been aggressively trying to move existing Windows 7 and 8.x users to Windows 10 via a free upgrade program that is ending July 29.
Ending the free upgrade is likely to have relatively little impact on the uptake of Windows 10, however, as few users get a new version of Windows via an update. They typically get a new version via a volume-licensing/purchasing agreement, or when they buy new hardware with a new operating system installed.