A reminder that Sugimori's official art is predated by the Red/Green sprites, which reflect what the original designs of the Pokemon were. If you look at their sprites here, it's extremely clear that there was never any mix-up, and the striking difference between Dragonite and Dragonair comes down a lot to how Sugimori chose to amend the designs.
I'm pretty sure that I read an interview with Atsuko Nishida once, where she stated that Dragonite predated Dratini/Dragonair as a design (which is reflected in internal data), and she made them to intentionally look quite different, as her preference were Pokemon that went through radical changes during evolution.