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You could probably get a phd in math using only textbooks with some variation of "introductory" in the title, so when dealing with math textbooks I recommend ignoring any words that normal people would interpret as indicating a certain level of difficulty.

From the intro:

> The mathematical level of these notes is deliberately low; nothing should be beyond a competent third-year undergraduate. But every subject covered here can be profitably studied using vastly more sophisticated techniques; that’s why this is advanced data analysis from an elementary point of view.

"Elementary" is a relative term in my experience.

The formula on 29 is the expected value of the quantity, (Y-m)^2. It is the average squared error under the distribution of Y incurred by a chosen value of m.

Note that the first sentence of the linked page says that:

> This is a draft textbook on data analysis methods, intended for a one-semester course for advance undergraduate students who have already taken classes in probability, mathematical statistics, and linear regression.

I think he means elementary in the sense of foundational.