Iirc Dumbledore's theory to why Nagini was made a horcrux was due to not only her presence during the murder of the groundskeeper, but that she had discovered Voldemort in Albania and been his companion.
In the same or a similar conversation in regards to the Horcrux, there was talk of how difficult and dangerous it was to turn a human into a horcrux. An animal would be difficult enough, which was what made Nagini interesting.
Those combined don't make this revelation a retcon, but it does shift what the understanding was from the original novels.
What this does help explain was Nagini at Godric's Hallow. Her being able to manipulate the corpse of Bathilda Bagshot to a convincing degree was always a point of confusion. I can't really buy all the Nagini was always clearly more than a snake, but that sequence sells it.