There is a portion of time where all of the lights at the intersection are red. The person you are responding to notes that as long as this portion of time is increased such that the total time that the intersection has for someone to clear it (i.e. - time that the light is yellow plus time that the lights are all red) stays the same then safety inside the intersection should not be impacted.
This says nothing about safety leading up to the intersection (e.g. - people slamming on their brakes).
I am worried about the case where the driver, distracted, looks away from the intersection for a second. Returning their gaze to the light, they see it red. They never observed it yellow, and thus they have no idea how long it has been red and whether or not it is safe to run the light. Perhaps this is why we have yellow to begin with--as an redundant means of clocking the time until the opposing light turns green.
This says nothing about safety leading up to the intersection (e.g. - people slamming on their brakes).