The title of the article is "Quantum Computing fin Rust, part 1", so the implication is that it is there to teach you about Rust, not to teach Quantum Mechanics which you already know.
It's not too much to expect an article entitled "What is a ket?" to explain what a Ket is.
Your point is arguable if it was instead "How to implement a Ket in Rust", and even then it's a bit vague. I left with the same feeling that it was a bit barebones in terms of definition.
