The GPU in question here, the PowerVR G6430, does not support OpenGL 4.5. It tops out at OpenGL 3.2 since it's a mobile part so everyone is using GLES.

The blog post only somewhat touches on it but I believe a key part of why this is so much faster is that it sounds like Vulkan is actually exposing tiled-based rendering. Note that PowerVR is a tile-based deferred renderer under the hood[1]. So if the app is doing the work of chunking draws into tiles instead of the driver trying to extract that from the OpenGL stream that's going to be significantly more efficient.

1: http://blog.imgtec.com/powervr/a-look-at-the-powervr-graphic...