Jellyfish venom doesn’t work through skin contact, but rather is injected through the skin via thousands of tiny harpoons. Victims do commonly die from drowning, but even more commonly the cause of death from box jellyfish stings was heart failure prior to anti-venom being available. Without anti-venom the chances of dieing are actually fairly good, with about 55 deaths in the last 15 years
The name fierce snake is one of the accepted common names for the inland Taipan according to Queensland Parks and Wildlife Service, along with small scaled snake and western taipan and wasn’t made up by the folks producing the TV show
Of course this is all irrelevant because when it comes right down to it the most poisonous creature, as opposed to animal, on Earth is the bacterium responsible for botulism if you want to use LD50 as your guage for most poisonous. Clostridium botulinum toxin has an LD50 of 0.001 mg/kg if swallowed and 0.00001 mg/kg if injected subcutaneously. That makes it by far the most potent venom of any creature on Earth and kind of makes spiders, snakes, frogs and jellyfish look a little lame.