Since humans are known for mating with anything even remotely like themselves, mixtures are to be expected.
In high school biology it was taught that a species is defined by the ability of a male and a female to produce fertile offspring.
A donkey and a horse make a sterile mule. Therefore they are not the same species.
A Labrador retriever can mate with a bulldog, as different as they are, and still produce fertile offspring. Therefore they are of the same species.
If ancient Homo sapiens mated with other hominids and produced fertile offspring, which in the case of Neanderthals they did, should the two groups be considered subspecies, not separate species?