On second thought, considering the time scales involved, post-urban would be a better standard for "modern" than would be post-industrial.
But even by that standard the Sentinelese are hardly modern! They are pre-urban. And they aren't the only ones either. So, to wrap my part in this whole digression up, my conjectures have been extrapolated from the behaviors of non-urban subsistence tribes.
OK, my point is that you are assuming that "modern" non-urban subsistence tribes show the same social norms as pre-historic humans. This is reasonable, yet not definate. But let's go with it. Does it preclude meaningful interbreeding barring other factors? I'd say no. Taking the women of other tribes and forcing them into your tribe, is a behavior of many "primitives". Thus Neanderthal women would have been expected to rear their ugly heads (so to speak) in the gene pool, which DNA study claims to preclude. That's why I'm guessing there were stronger factors involved.