I don't think that is a fair requirement. The baseline is that eglSwapBuffers cannot be spec-correct to start with, since eglSwapBuffers cannot swap buffers in the first place. The way I understand is that the function is supposed to swap buffers to present, and in the process of doing so, it will act as if glFlush had been called. Since we can't present here in the first place, I don't see the point of introducing a potential GPU+CPU sync point that is not observable except by possibly hitting performance if glFlush was implemented as a full sync point on some device.