I don't think it is the proprietary driver - in the video they say "mainline Linux with DRM/KMS graphics acceleration" - afaik ARM's proprietary driver doesn't use KMS.
Bear in mind that modesetting is handled by the display controller, which is a separate unit not supplied by ARM with its own driver (in this case, an open-source one that's now part of the mainline Linux kernel). This is why the lack of open source Mali drivers is less of an issue than PC users might assume; it doesn't affect the availability of unaccelerated 2D at all. The display controller is even mostly documented in this case.