The retina is basically an extension of the brain as well, no? If not, what is the key difference? Do you have a link talking about the atmosphere?

But the retina is not exposed to the outside world. It's got the rest of the eye between it and the air.

Ok, but is brain tissue uniquely infectable compared to eye tissue or something? The ultimate worry isn't that your olfactory bulb gets hurt, it's that the infection reaches deeper into the brain, and I don't get why it's easier to do that from the olfactory bulb rather than from the eye.

the conjunctiva of the eye is different tissue than optic nerve. there is fat, connective tissue, and muscle surrounding the eye.