That is a diagram for identifying pit vipers. There are many, many venomous snakes in the world, including I believe all of the most deadly ones, that are not pit vipers.
The advice is particularly unfortunate for Australia, as not a single one of Australia's 10 most dangerous snakes are pit vipers. All 10 are instead elapids.
The origin of this graphic is almost certainly English-speaking North America. For much, if not all of that area, the only venomous wild snakes are pit vipers. As a kid in Oklahoma I was given these exact instructions verbally for identifying poisonous snakes*. This simplification is possible in the USA because the only venomous snake in the continental USA that isn't a pit viper is the coral snake, whose range is limited to the Southeast, is rarely seen even there, and is generally not aggressive.
* - I am repeating the term that was used at the time for these snakes when said people were talking about them. I know its fashionable these days to maintain that "poisonous" doesn't really mean that. For that reason when I'm trying to be technical I'll instead use "venomous". But consider that language is arrived at by consensus, not dictate. If you have to explain to everyone you meet what a word means, you should consider the possibility that it isn't they who are wrong.