From the text: “Also, remember: “leaves of three, let them be” — groups of three leaves being the sign of poison ivy, sumac, and oak.”
Poison sumac has 9 to 13 leaflets usually on its compound leaf.
www.duke.edu/~cwcook/trees/tove4969.jpg
You are of course correct, and I notified folks to make sure that the column was updated. The explanation I heard was that a copy editor down the line had a moment of brief confusion, which rippled through the fabric of reality and caused the vacuum to spawn forth an extra word (and a corresponding anti-word, which ended up over at Fox News and ate a hapless web designer, if I recall correctly). Hazards of the business, they say.