Let’s say we left one single very smart guy (not necessarily with the knowledge: they may be able to understand hard stuff when taught it, but not know it already) alone on a copy of the earth. That person is also immortal. Could that person, by themselves, gain back all knowledge, maybe also experimental, or even surpass that is already available to us right now, before the planet gets inevitably engulfed by a sun turning red giant?
I think be smart isn’t enough to find the motivation to do something. More harder is the goal and much motivations are need it. Human race actually have evolved during crieses and inventions was maded to fix immediate needs, like market at beginning of trades in the babilonian cities. One single guy left alone even with infinite time can do nothing, or actually, you say about the sun turning a red giant, he will find the way to build a rocket probably a couple of years before that happen, just in time to flew like a rat motivated by survival instinct.