You don't need imaginary numbers to define the mandelbrot set. Start with point (x[1],y[1]). When you do the iterations, x[next] = x[previous]^2 - y^2 + x[1] and y[next] = -y[previous]^2 + y[1]. And the image shown is most definitely a rendering of the mandelbrot set. Other than that, you're right that complex numbers and cartesian coordinate systems weren't used when this drawing was supposedly made. The article even claims the monk performed 70 iterations on each point to test whether or not it was in the set! It took my Commodore 128 almost 24 hours exactly to do a 320x200 picture that only tested points for 12 iterations. The implication is that the monk performed 1,008,000 calculations just to create the 120x120 grid. April fool.

Oh, and wouldn't it be nice if slashdot supported the tag?

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