If they happened (emitted) at the same time that would be true, but they don’t.
Because the sequence of nuclear reactions gamma rays happen first and visible light is next.
Unfortunately for us if Earth is ever hit by a nearby supernova we will be dead by the wave of gamma rays before we see the blinding light.
OK ... didn’t know that about the sequencing. So we get blasted by gamma, then the cockroaches standing on our corpses see the big flash. Yay, cockroaches!