I made no such assumption. You don't need an event horizon to dilate time, just a massive gravitational field, and something resisting it.
When the sun collapses either the outer layers experience a repulsive force of some kind from below (a bounce perhaps, which is already theorized for supernovas, or maybe neutron degeneracy), or the outer layers accelerate to nearly the speed of light, either way they dilate time, and from the POV of an outside observer take an infinite amount of time to collapse.
Because of this dilation the rate of light output (as seen from outside) will drop and the sun will "turn off" - i.e. it will look [almost] identical to a black hole, except without the physics breaking singularity or event horizon.
One visible effect will be extreme red-shift of the light, which will look (to us) like a very distant object hubble-red-shifted by distance/time.
