Interesting read, especially as I had never noticed that it was part of Apple’s UI guidelines. I always assumed it was a stylistic difference with the US favouring Title Case vs Sentence case for the rest of the world. (A massive generalisation, I know.)
I remember trying to explain this very stylistic difference to American colleagues when I needed to make a case for why sentence case was more appropriate in a product for the UK market. At the time, I shared links with reputable news sites from the US and the UK so they could appreciate the difference with real world examples.
I’ve found that having a clear typographic hierarchy helps to distinguish a title from body copy. Perhaps that’s why I’ve always preferred sentence case, as I find title case more difficult to read.