It holds true enough for North America, but not in other parts of the world, e.g. Africa. So yes, it should have a disclaimer.

This is obviously not true as many of the dangerous snakes in Southern Africa have paired subcaudal scales including the Black Mamba, Green Mamba, Cape Cobra, Vine Snake, Boomslang and Puff Adder to name a few.

Source: africansnakebiteinstitute.com/…/harmless-vs-venom…