Its a good observation, and I could not see the 99.7% figure anywhere in the original scientific paper. A few percent (up to 5%) of genetic material seems to have flowed in either direction. (H->N and N->H).

What it might mean, though, is that the sequence identity between H and N = 99.7%. That is to say, 99.7% of nucleotides are identical in the H and N genomes, while 0.3% have changed. It is those 0.3% that can act as markers for genetic exchange: At 0.3%, we can say that ~1 in every 300 nucleotides has changed in H with respect to N, which in a 3 gigabase genome is ~10 million nucleotides.

These 10 million nucleotides can act as identity markers for genome-of-origin: Now if some modern humans, say Africans, have 9.5 million of the H (human) markers and 500,000 of the N (Neanderthal) markers, we can say that 5% of African DNA is of N (neanderthal) origin.

This seems to be typical crappy scientific journalism - the same kind that turned a virus of legitimate concern (COVID) into an authoritarian nightmare. There are a lot of failed scientists in the world, MediaLens is run by a couple.

Enoch Powell is quoted as saying “Every political career ends in failure”. Same goes for scientists, who keep going until they can no longer get grant funding. Then they do other stuff….

- Shyaku