What does “there is some truth to that” mean?
Technically, the writer is saying that there is some truth to the idea that some people are born lacking in courage and you can't change who you are. Which they later directly contradict.
I think what they mean is that they recognize that it's not easy to "change who you are" and become more courageous once you've lost your courage.
And if “some people are not born courageous, all of us are”, why did the
writer say “everyone who is not courageous anymore has obviously already
The writer is saying that it's not true that only some people are born courageous. They're saying that everyone is born courageous, and people who are no longer courageous have changed who they are; which, they're saying, demonstrates that people can change who they are, so, there is always reason to hope that a person who has become cowardly (or, well, not-courageous) can change who they are and become courageous.
This is without a doubt not the place for airing personal opinions on extraneous subjects, but this is a point on which I suffer a strong urge to air an opinion.
My take is that people can't change themselves, but others can.
Identical twin studies have fairly well established that people have incredibly strong genetic predispositions. So in a very real way, you can't help yourself, your behavior will be what your genes dictate that it will. But your genes don't say that you will behave a certain way no matter what. They merely say that you'll behave that way in the absence of sufficient external pressure.
Can your husband quit smoking? Nope. He is utterly physically incapable of it. Unless you leave and tell him you'll come back after he has quit and has convinced you that he'll never smoke again. Then all of a sudden he can! It's a miracle! It's like flipping a switch.
Can rapists avoid raping? Nope. Unless we castrate rapists. Then you bet they can! It works like magic.
Are people responsible for their behavior?
Does it matter?
Not at all.