I think you're confusing two things. While in PC-land the GPU may (or may not) be involved in video decoding (stuff like Intel's VAAPI or nVidia's VDPAU), in ARM SoC-land, the GPU is quite often another beast from the part of the chip that decodes the video. The GPU, of course, is involved in rendering all those 3D Android games you play. But for showing stuff like so-called H265 video, an Android settop box would rely on a custom hardware video decoder separate from the GPU. This is quite similar to the way some PC chips have built in AES support.
This makes sense even if I'm to lazy to include links to back up my post. Everybody knows how GPU's are used for 3D games and those horrid wobbly desktop effects, while videos, whether they're plain MPEG or H26x, are strictly 2D. Intel's power-hungry CPUs can effectively brute-force the higher end video codecs like H265, while the lower-power Android SoCs require a hardware-based solution.