Is it correct to regard archeaic humans (i.e.n Neanderthals and Denisovans) as distinct species to Homo sapiens?
This question highlights the different species concepts. 

If you go by the reproductive species concept (a population of actually or potentially interbreeding organisms), then yes. Apparently, the populations were interbreeding.

If you go by the typological species concept (morphologically distinct), then no. Homo neandertalensis and the "Denosovian" are morphologically distinct enough to be recognized as such.