Not from a single strand of DNA, no. What they mean is that the child of a Viltrumite and human will be 99% Viltrumite in terms of genetic makeup.

You can’t edit the genome of a multicellular organism

Except you sort of can to a degree. Somatic cell gene editing is an emerging field with the potential to lead to treatments for genetic diseases in the future by doing exactly that. There’s also some funky shit being done with adult stem cells that could lead to some pretty interesting shenanigans down the line.

Realistically there’s no telling how well or how poorly that statement will age in the next few decades considering our understanding of this area of biology is only going to grow. I doubt superpowers will ever happen of course, but large scale modification of genes in adult humans isn’t exactly out of the question.

Aside from changing the order of some events, overall it’s nearly entirely faithful so far. The changes and omissions that have been made have all been for the better in my opinion.