Yes, a theory is at its heart just a guess -- which is why we can have lots of theories! That said, what matters is finding the theory that best explains the facts as we know them. You might like this better explanation by Feynman:
Feynman on Scientific Method
Perhaps where we differ is on whether or not the biblical reference to 600 years should be taken as a reliable report of lifespan at the time. From my standpoint, it certainly isn't reliable enough to posit different genetics just 10,000 years ago.
There are lots of possible theories.
In fact I might have to concede to you that it is essentially impossible to prove that an theory is wrong beyond any shadow of a doubt. But, again, the real challenge is to identify the one theory out of the bunch that does the best job of explaining the facts as we know them. That best theory is just that - the best theory. We are never going to be able to prove that it is 100% correct and have to be open to the idea that new facts will require continuous refinements in our best theories. We can aspire to being 100% right, but we will never get there. But making choices based on the best theories of the day gives us our best chance of being right.
Beliefs based on faith are by definition ones that are held to be 100% right. Theories manufactured to provide pseudo-scientific cover for biblical contradictions with everyday facts are aimed at preserving that faith-based belief at 100%. Or so it seems to me.