Lions and tigers can mate and have fertile offspring yet no one would consider them the same species [1]. Actually there is evidence that the offspring of ancient African and Neanderthal matings [2] followed Haldan's rule [3].

If this discussion was about anything other than humans there would be no question that Ancient Africans, Neanderthals and Denosovians were anything other than different species.

1. https://en.m.wikipedia.org/wiki/Liger

2. http://www.cell.com/ajhg/abstract/S0002-9297(16)30033-7

3. https://en.m.wikipedia.org/wiki/Haldane%27s_rule