I think the belief is more grounded in them wanting it to be true. A faith-based belief. In the video he refers to ancient Egyptians and lost hidden knowledge. I would categorize it as a form of pseudo-scientific spirituality.

I don't think the belief starts from a rational basis. The guy likes snakes, has a rational basis for developing immunity through self-envenomation. At some point he might want the project to bring even more value than that. "Ageing" is a nice target because it is an aliment that affects us all and for which we have no solution. The rationalization would then come after hoping that this is true.

As how to rationalize. I am not sure but I have a guess. I often I see rationalizations for this kind of claim to hinge on the idea of adapting the body to survive some form of stress. The stress-adaptation arguments draw an analogy to the concept of working out and vaccines. You stress and break your muscle cells, your muscles recover and become stronger. You inject venom or small quantities of a virus and your body becomes better able to fight it.

Many pseudoscientific ideas generalize the concept of stress adaptation and apply it to other situations even if the data is not available (or, worse, contradicts it). For example: some people will actively expose themselves to low doses of radiation in the hope that this will train their cells to repair DNA damage by up-regulating DNA repair genes. The idea is that they increase their resistance to ageing. For snake venom: my guess is that they imagine that snake venoms will stress some ageing-relevant pathway that will somehow train adapt the body to resist such stress, and this way the body is more resistant to ageing.

What makes bodybuilding and vaccines scientific but radiation dosing and snake-venom-for-ageing pseudoscientific is that... the first two are supported by a lot of data! The others are hypotheses that might be worth testing, but skipping all steps and going right to self-medicating due to holding a strong belief without data is pseudoscientific.

Snake venom gram for gram is a lot less toxic than some other substances we consume. (For fun, look up how much a lethal does of caffeine is) I can understand the logic of testing snake venom for medicinal properties, but injecting yourself with it as your test is the jump over the line to stupid.

Mr Friede's motivation was to develop better therapies for the rest of the world, explaining: "It just became a lifestyle and I just kept pushing and pushing and pushing as hard as I could push - for the people who are 8,000 miles away from me who die from snakebite".

He didn't do it for anything resembling anti-aging

Edit: oh, just read the last of the post mentioning the dude doing it for anti-aging...yeah, that seems messed up

someone else who did something like this lived to be over 100. maybe that's where the idea came from?

May be just a hook, to make things more interesting for themselves.

Though, i would choose something like "it makes you invincible" or "I'll be able to communicate with snakes"