"unicode bytes" aren't a thing; bytes implies encoding (and subscripting yielding <=0xff), otherwise it's "codepoints" (and subscripting yielding an int somewhere on the unicode planes).
String and str are guaranteed to be valid UTF-8 encoded Unicode strings. If you’re wondering why UTF-8 is the standard encoding for Rust strings, check out the Rust FAQ’s answer to that question.
That's spot on. Please add this to the first part, too; "... buffer of UTF-8 encoded unicode bytes". or even just "encoded unicode string." It will be clear what is (and is not) meant.
Otherwise nice article! Even understandable for someone with no Rust experience.
As a kinda funny aside, I also wrote the linked-to FAQ answer. Took a number of drafts to get all the fiddly Unicode terminology right.