Author of the original article here. I feel your pain, but again, it's just a different set of conventions. Mathematicians are used to f(x) = ... kind of notation. Once you get used to it, it makes total sense. That particular one I got used to ages ago. f(x) is the same as a function in your code. It takes and argument, x, and returns some value.

Often specific symbols have implicit meanings, like theta θ is pretty commonly used for some angle, r is often used to mean a radius. So you'll often see something like "r sin θ" with no explanation. At first it's meaningless, but once you know the conventions, it's crystal clear. It's considered so basic, that nobody would waste the space explaining it. Same is if you're reading something about code and something says "const float x = 0.1" or something. The author is probably not going to go into an explanation of what a const or a float is or what x means. You're expected to know.

So what I like about the book is that helps someone without knowledge of all these conventions to begin to understand them.