I$ is the u̶s̶u̶a̶l̶ ̶c̶u̶t̶e̶ ̶n̶a̶m̶e canonical shorthand.

the point being that lower-case i is an Apple-ism, while the correct casing is upper-case I (I've also seen all lower case when used informally, e.g. icache, iram, etc.)

I have trouble distinguishing between uppercase i vs lowercase L in my system fonts, and am too lazy to change them, so I appreciate the author using iCache for readability.

Not really a legitimate reason to standardize such a weak idea though.