Mark covers the topic of the arch as best one can, but the general public seems to get lost many times in his messages.
There's many gray areas that apply in different scenarios.
We see some people that seem to have no spine doing an arch that creates a vision them snapping in two and think that's wrong which leads to arching is wrong.
Now you have the other end of the spectrum where lifters are trying to arch an have almost zero flexibility in there spinal column.
Why is it good to increase flexibility in other parts of your body but not your spine?
Another variable is different lever lengths. Some people have very long arms and a thin chests putting their shoulders in a very compromising position at the bottom of the bench press movement.
Weighing these factors together one can better filter what is beneficial and what might be higher risk.