Quick, simplified lesson...
There are three groups of venomous snakes;
Solenoglyphs, with 'hinged' front fangs (vipers)
Proteroglyphs, with small 'fixed' front fangs (cobras, mambas, kraits, sea-snakes etc)
Opisthoglyphs, with enlarged rear fangs (Boomslang, Twig Snake, Montpellier Snake, others)

It would be wrong (and dangerous) to exclude the Opisthoglyphs from the 'truely' venomous snakes. These snakes are not border-line venomous, like the Garters. These are true venomous species, complete with venom glands, venom ducts and enlarged, grooved rear fangs for delivering the sometimes extremely potent venoms. Both the Boomslang and the Twig Snake are known to have caused human fatalities.