A few links have been sent in to our news tip box with this page, which reads, "Open Source Mali-200/300/400/450 GPU Kernel Device Drivers." While the page mentions open-source drivers, it's only about the kernel portion of the driver and it's always been that way with ARM -- and most other ARM-based graphics vendors. The kernel portion is open, the user-space components are closed. Without an open user-space, having an open kernel driver is only of limited use, and will not be accepted into the upstream Linux kernel.
ARM makes available the kernel driver for some easier porting to different kernel releases, but it doesn't really allow users much. It's the user-space binary blob that's feeding the kernel driver all of its magic when running OpenGL ES, etc. The Lima driver that's been in the works for a few years by Luc Verhaegen is what's being attempted as a reverse-engineered user-space driver for Mali to replace this blob.
Qualcomm was in a similar boat when Freedreno started for open-source Adreno support where the Freedreno Gallium3D driver was written against the open Qualcomm kernel blob interfaces and over time is when Rob Clark of Freedreno also ended up writing his own proper DRM driver (called "MSM") that's since been mainlined.
So long story short, that ARM Mali Developer Center page isn't new, and their open kernel driver isn't worth too much without an open user-space driver. The latest Mali 400/450 driver release is even from last November.