Mixing between modern humans and neanderthals

Remains in Bulgaria show that in the Initial Upper Paleolithic (IUP), modern humans had Neanderthal ancestors as recently as 6 generations before[@hajdinjak2021Initial Upper Palaeolithic humans in Europe had recent Neanderthal ancestry]. These are individuals that lived around 46ky and 42ky before the present.

Surprisingly, individuals that lived thousands of years apart show Neanderthal ancestors between 6 and 7 generations before. If 6 generations account for around 100 years, this should point to the continuous mixing of Neanderthals and modern humans.

The amount of Neanderthal genome found on the individuals in Bacho Kiro cave is around 3.8%, which is almost exactly

The average fraction of Neanderthal genome in present day humans is around 1.9%.

I wonder what is the role of introgression to define the percentage of modern and neanderthal genome. Moreover, if modern humans mixed with Neanderthals often, wouldn't it be possible to say that present day humans are descendant of both?