Perhaps the linked article was changed after the question was posed here, but the caption under the image states,
Wasiczek’s backgrounds are as interesting as her flowers. Here, she employs her mirror lens’ distinctive donut-shaped bokeh. Shot with the Nikon D300 and a Rubinar 300mm f/4.5 mirror lens.
A catadioptric, or mirror, lens essentially acts as if it had a filter with an opaque disk blocking the center ~1/3 of the lens. Because that’s exactly what is going on — the back of the 2nd mirror in the folded optical path blocks the light from entering the center of the lens.
I believe you also get this kind of bokeh if you use a mirror lens.
(Random google results:)