The author unfortunately has a misunderstanding of equality.

Quoting him, "The equal sign is another mathematical concept that’s often misunderstood. It means, of course, that whatever is on either side of the equal sign is equivalent." This is actually wrong.

The equals sign (=) is a shorthand for stating not that the two sides are equivalent, but that they are the same (i.e., they are equal).

If they were just equivalent, we would use another sign, like ≡ (unicode U+2261).