[parennoob]

> Mathematics proper, as in the derivation of theorems from axioms by means of proof

That is an extremely narrow definition [1]. It is acceptable to me as someone who works in the intersection of technology and mathematics -- but for the general public the word implies a wide field of study, exploration and usage.

For example, on Wikipedia "Mathematics (from Greek μάθημα máthēma, “knowledge, study, learning”) is the study of topics such as quantity (numbers), structure, space, and change."

So all the author is saying that the residents of Benin City seem to have studied fractal patterns in some form and used them in art.

------

[1] Although, reading your comment more carefully, you did qualify it with "mathematics proper". I guess my comment's whole point is that it's often "mathematics improper" when used by a popular columnist.