This was the actual codename for the project. Leaked ISOs of these builds featured a hybrid interface—mixing the Windows 8 "Live Tiles" with a returned, experimental Start Menu.
Microsoft’s jump from Windows 8.1 directly to Windows 10 led to widespread speculation. The most credible theories include: Why did they jump from Windows 8 to Windows 10 and skip 9? Windows 9 Beta Iso Torrent
These early technical previews were often referred to by the press and enthusiasts as "Windows 9" before the official naming announcement. This was the actual codename for the project
Technically, "Windows 9" did exist in the form of early development builds for what eventually became Windows 10. Windows 9 Beta Iso Torrent