There's a difference between sub-species and separate species. The ability to produce fertile offspring is a key characteristic in defining species as separate. Sub-species can produce fertile offspring.At least some of the offspring seem to have been fertile. Skeletal remains have been found showing features that appear to combine features of both modern humans and Neanderthals. DNA testing suggests hybridization between the two “species” did happen. Species is a blurry definition in this sense since at least some of the offspring were fertile. There are many instances of inter species breeding and generating offspring and if closely related enough leading to fertile offspring.