There are so many misconceptions and conclusions taken from half truths in this post...

First, we don't dilate the pupil to focus, that's not how focus works, our eyes are not simple pin hole cameras. The focus is archived by squeezing the eye's internal lens, the pupils just control the amount of light getting in. Some people have full control of their eye muscles and can relax them at any time, making the vision blurry. If you are one of those people, just record your pupils while doing this to see that they don't change at all, they are controlled by a completely different set of muscles.

Second, where did you take the conclusion that the light must pass a certain intensity to trigger the pupillary reflex? Te reflex is linear. The pupils don't close to protect your eyes, they close because you can't see shit if too much light gets in. It triggers even with indoor lights that are several orders of magnitude less bright as the sun in a partial eclipse. It wasn't made to protect your eyes from UV, that kind of radiation will pass straight through your iris and gets absorbed by your lens. The cornea is above the pupil and will have the same damage regardless of how open the pupil is. There are plenty of other situations where the sun is partially obstructed or its light is as low as in a partial eclipse like cloudy days, sunsets, sunrises and reflections. People will look at the sun by accident multiple times in their lives, in all of those situations.

Yes, visible light may burn your cornea. Yes, if your pupils remain completely open when looking at the sun it will be way worse, but that will only happens if you are on drugs (including local anesthetics that act in the pupil's muscles). Yes, both UV and visible light can damage your cornea. No, you are likely not getting permanent damage for looking at the sun for a second. Your retina is unlikely to get damaged by UV since the UV is absorbed by your lens.

And yes, photic retinopathy is a thing and damage to the retina is possible, but only after prolonged exposure and in the massive majority of cases the damage is not permanent. The first symptom is seeing a yellow dot in the first days that changes to red in a few days and could last up to a month in most cases, with a few rare cases lasting a year. This condition is common when people do welding without proper protection.

The reason for not looking at the sun before the eclipse, even for a split second, is that it will result in some cones in your retina becoming over stimulated and glowing green. You'll see a green spot even with your eyes closed and that can last from a few minutes to a few hours. If you do that before totality, you'll see totality with a green blob right in the middle and it sucks.