A flag at night makes little difference if it's barely visible or not. I think you meant to say "without mandated lights and reflectors".
I believe mikepurvis's comment was that a lot of responsibility disappears, not all responsibility.
It is the driver's responsibility to not drive faster than can be seen. There can be cyclists, certainly, or someone with a seizure or heart attack who ended up on the ground, or pets, livestock, or wild animals. People do hit deer in the middle of the night, when the deer freezes up and the cars are going too fast to break in time. To reflect my Florida upbringing, people do accidentally drive into sinkholes which appeared in the middle of the street, again because they were driving too fast to break in time.
They drive fast because their experience mislead them. They think that years of driving at night at a given speed, without problem, means it's safe to do so. Think of how some drunk drivers will keep doing so, on the belief that they can hold their liquor enough to handle a car. Until the day they cannot.
A driver who hits a cyclist - even one without lights - was going too fast to control the car safely.
This of course has limits. But the car can cause much more damage, so the operator of a car should have higher responsibility than that of a bicycle.
So no, it's not that cyclists are doing a less safe thing, it's that motorists are doing a less safe thing.