It is probably trite to question whether they could have but we must also enquire whether they would have. Stating the obvious, they could have subject to the caveat that they only would have if they were members of the same species as different specious refuse to mate.
How and why did these other species of humans disappear? The answer to the “same species” question will determine which theory takes primacy: Interbreeding or Replacement?
Main picture: Theoretically the DNA of the Neanderthals & Homo Sapiens should be so far apart that they never should be able to interbreed. However is that correct?