I don't think it changes anything at all. It actually adds to it.
Deathwing wanted to create a flight of dragons that could wield the powers of all 5 flights, which Nefarian continued, and the Twilight Father succeeded. Chromatus was the only "true" Chromatic dragon that didn't (and couldn't) die. The power of Azeroth's magic pouring through the Nexus was required to make the only true Chromatic dragon. Even the other Chromatics we see use limited powers, and we have never seen one older than a drake. Plus, they're all pink or purple.
Drakthyr are draconic-infused mortals. They were, basically, the first step toward making a Chromatic flight. But, they aren't "true" dragons. They're still part mortal. They can have many different colors for their scales, including silver and purple. I haven't even noticed if they have a Chromatic magenta color for their scales or hair.
It fits into the narrative perfectly. These were the first experiments towards a Chromatic flight. And while the Drakthyr were a successful experiment, they weren't the entire result that Deathwing wanted. Using the information Deathwing gathered to create the Drakthyr, Nefarian later on created the Chromatic dragonflight, but it still wasn't perfect. They didn't entirely have the powers of all 5 flights, and most died from birth. Then, with that data, the Twilight Father created the only one true Chromatic dragon, with 5 heads for each power.
Plus, we see a few different dragon-people that were combinations of mortals and dragons that were Nefarian's experiments.
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Kyrak was one of the last living experiments of Nefarian's. He was forced to help the Iron Horde.
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Maloriak was a human alchemist that was kidnapped by Nefarian and fused with the corpse of a Dragonspawn.
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Tarvus the Vile can be found in Twilight Highlands.
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Chromatic Prototypes assist Nefarian in his Cataclysm appearance.
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Obsidian Wing-Spreaders appear in Island Expeditions among the Black Dragonflight (with one Chromatic drake).
-There were also failed dragonman experiments, like
Aberrations and
Drakonid Monstrosity.
This suggests that Nefarian was trying to replicate the Drakthyr, but he wasn't entirely successful.
So, it fits in the lore fine.