The author worked on both compiler (first Rust as a summer intern at Mozilla, then Swift at Apple and now back again on Rust at Mozilla). They're one of the best placed people to talk about the differences between these two language.
More accurately I worked on stdlib stuff for both, with a focus on collections. It's just that this naturally pushes you into minoring in language design and majoring in the low-level details of the language. Plus it's hard to not pick this stuff up if you have to hang out with compiler people all day.
