"X is part of human nature" can mean

-- X cannot be changed

-- X should not be changed

-- X has particularly deep connections to human psychology

It always and only means the third of these (with minor variations, e.g. theists will talk about souls created by God). The first and second are then drawn as implications of the third.

In your quote, "death is part of human nature" has the second meaning

It has the third meaning, as you could have discovered by consulting the sources I gave. The whole purpose of the authors of that report was to address the question, if various enhancements to human bodies, of which life extension is one, can be made, should they be made?The "human nature" argument presented there was based on our mortality having "particularly deep connections to human psychology".