> Generally Neanderthals are pointed out as an exception to cross species fertility since...
There's no such rule and Neanderthals are not notable as an exception. Fertility is just a very rough proxy for genetic distance, which is correlated to our arbitrary "species" buckets but by no means a real line or hard rule. Many, many reasonably closely related species can interbreed, like jaguars and lions. Most of homo that had the opportunity could probably interbreed.
There's no such rule and Neanderthals are not notable as an exception. Fertility is just a very rough proxy for genetic distance, which is correlated to our arbitrary "species" buckets but by no means a real line or hard rule. Many, many reasonably closely related species can interbreed, like jaguars and lions. Most of homo that had the opportunity could probably interbreed.