I can't see the paper on that page, just the image, is there more to it than just the image and the claim?
What snakes have they checked?
edit: I think this is really about identifying rattlesnakes, more than about identifying venomous snakes in general. There's two counterexamples on the "anal scale" page of wikipedia
https://en.wikipedia.org/wiki/Anal_scale
the undivided end on an elapid venomous snake, and the divided on a colubrid
I think they should use matrix