This is a good text, at an upper-div/grad level, of fundamental neuroscience with all sources cited.
That particular connection is straightforward to do in humans. A Golgi stain to the rector muscles/ON and dissection in cadavers would be sufficient to trace the reflex to the SC and then another Golgi stain to that area to get back to the optic nerve. I'm unfamiliar with the toxicity of Golgi stains, but it may be able to be done alive.
Also, the visual systems to the brain-stem are remarkably conserved through evolution. I would not be surprised to see this connection in lampreys. That any significant percent of humans lack it would be a hell of paper.
Blind individuals usually have these reflexes too (like Stevie Wonder): https://en.wikipedia.org/wiki/Blindsight
I was able to check a bit and see no citations:
"The human brain contains a huge number of these cells, on the order of 10^11 neurons, that can be classified into at least a thousand different types."
That 10^11 number is out of thin air. How was it determined? That is what a citation is for.
>"I'm unfamiliar with the toxicity of Golgi stains, but it may be able to be done alive."
No, the gogli stain is very toxic. It depends on a precipitate forming in "random" (no one knows why) cells. Also I see no reason it couldn't spread from cell to cell (via gap junctions, etc) so that method isn't too convincing.
>"Also, the visual systems to the brain-stem are remarkably conserved through evolution."
You can remove a rat's cerebrum and have it stay alive and keep doing stuff:
"Cage climbing, resistance to gravity, suspension and muscle tone reactions, rhythmic vibrissae movements and examination of objects with snout and mandible were difficult to distinguish from controls."
Rodents are much more reliant on their brainstem than humans, I wouldn't be at all surprised that there are large differences. In fact, there's been a long debate about a similar claim regarding the cortico-spinal tract:
"Direct connections between corticospinal (CS) axons and motoneurons (MNs) appear to be present only in higher primates, where they are essential for discrete movement of the digits. Their presence in adult rodents was once claimed but is now questioned."
If you really have a problem with Kandel, use email. Most authors of these types of book NEVER get any email about them and would be thrilled to have some interaction with a reader.
"The optic nerve is directly connected to the superior colliculus"
It seems very factual and set in stone but I bet if you read the primary literature there will be variation and doubt. If you read my last ref you will see they claim direct connections between CST and motorneurons in rats of some ages but not others. Perhaps this optic nerve claim was made based on using animals of a certain age, so it won't generalize. Who knows? That's why there should be a citation.
tl;dr Current textbook practices promote false certainty, and I don't think it is helpful for learning about a topic.