Chromium (and Firefox) do GPU accelerated raster as well as composition. That page you linked is about a very specific case of wanting to get the pixels back to reuse in a different context. In that situation doing the work on the GPU and then copying back to the CPU may be slower than keeping it on the CPU the whole time.
suggests to me that the glyph rasterization (which is the CPU-limiting factor for text rendering) in WebRender (which is the new FF 93+ GPU-accelerated rendering engine) is implemented in Rust and to be run on CPU. On Linux it appears to use FreeType (which is CPU-only):
