No combination of Dragons was turned into a chromatic flight before. The Dracthyr are Dragonkin which still allows it to not be a retcon. Provide the passages of lore you keep referencing so we can see the exact terminology used.
Here is the write up from WoWpedia. It might not be direct lore but I think it covers the point sufficiently. All it says is about creating a Chromatic Dragonflight and not Chromatic things at all. As far as we know the Dracthyr are not the same as a Dragonflight so nothing is a retcon for them to be possible. They are the precursors to a dragonflight. So Nefarian could have been expanding on that knowledge.Nefarian was obsessed with following in his father's footsteps by achieving what his father could not. He has been experimenting with the blood of all of the various dragonflights to create a chromatic dragonflight of unstoppable warriors and unspeakable hybrids of both red and black dragons
It could even simply be Deathwing didn't know if his experiment was a success because the island was sealed before it could be known. There are a lot of different ways they could go that don't require it to be a retcon.