This assumes that the fundamental nature of human beings can't be changed, which is the presupposition that genetic engineering denies.

The entire point of genetic engineering is to try to engineer people at the cellular level. A fanciful example would be modifying human metabolism to be more similar to those of birds that consume most of their calories in the form of simple sugars. Humans can't eat a diet of 100% sugar and remain healthy, but other animals can. It may be possible to change that fact, if we know how to edit our genes.