This is because the brightness of the cluster refers to its INTEGRATED total light from its constituent stars. You could not see the individual stars, but would see a diffuse glow whose surface brightness equals a single +8m star. But because the light is spread out over a large region of the sky, its apparent contrast to the background sky is probably so low that the sky brightness is killing you from your observing site. The 9th magnitude stars you see have their light concentrated far more and they literally 'punch through' the sky background.