If modern humans have Neanderthal DNA (as proven recently) then that means modern humans could produce viable offsprings with Neanderthals. Then why are Neanderthals considered a different species?

https://www.quora.com/If-modern-humans-have-Neanderthal-DNA-as-proven-recently-then-that-means-modern-humans-could-produce-viable-offsprings-with-Neanderthals-Then-why-are-Neanderthals-considered-a-different-species