>>512837784 (OP)
Because they have enough fragments of the old history to know it's possible using materials science that our civilization should be able to recreate except there is a barrier of falsification obscuring the whole picture. Things were done in the past that we should rapidly be able to emulate. The advanced molecular physics sims will eventually crack it by brute force. Examination of ancient remains and artifacts will yield all sorts of hints. The ancient elite did it, that's what made them elite. Everything that exists today is a pale shadow. I've never seen more than rumor and myth to indicate anyone was able to preserve the science. The dark age eradicated it even from the upper class that rose to power on the desolating waves of fundamentalism.
These modern immortal dudes are just scratching the surface of biohacking, technically you're just playing damage control if you weren't conceived in a biohacked womb with special hormone parameters to play with epigenetic switches and accentuate different growth cycles. The real immortals were being dosed as they were built from cell 1. Imagine your zygote was on roids compared to just starting to mess with your hormone profile right now, for a crude example. You can even use specific toxins to damage different organs in the endocrine system to give another example of the degree to which biohacking was explored in ancient history. These chemical alterations could also be accompanied by surgical intervention and physical reshaping with the most well known example being headbinding.