Pulses of neutrinos released before the “explosion” Just a guess I don’t know what kind of convulsions a collapsing star goes through. The electromagnetic spectrum is much more than visible light.
Yabbut, in the same medium, vaccum in this case, it all propagates at the same velocity. I would guess precursor events released the neutrinos first.
Above a critical temperature threshold within a star, energy is diverted to production of neutrinos from the photons. The majority of neutrinos are not prevented from escaping with that energy. The photons (light energy) which support the stars bulk against gravity are now missing.
In a fraction of a second the core implosion’s shock compression results in conditions which covert some matter into elements more dense than iron, a pure neutron inner core which often continues to a black hole state, and energy release sufficient to explode the outer core layer away to free space. Light from this core explosion process is delayed in travel by the amount of time required for the debris to thin, such that the light trapped inside the cloud can escape.