One thing I've noticed about middle starter evos, at least recent ones, is that they tend to be much more similar to the first stages than the third stage. For example, Quilladin and Dewott share more characteristics with Chespin and Oshawott respectively than Chesnaught and Samurott. The middle evos do give hints about what the third stage will look like, but it's the third stage that really takes that drastic turn design-wise.

The Alola middle starter evos do the same, especially Torracat and Brionne.

Dartrix keeps its theme of being a ranged attacker. It also shares the same oval-shaped specks on its wings as Rowlet3.

Torracat seems to have developed muscular limbs and its paws have become thicker like Litten3.

Brionne's frills have taken on a more feminine dress-like appearance. The white portion of its skin has extended further, which points to Popplio3's white skin.

So while they do not look exactly like the leaked final evos, they share enough characteristics to make them seem plausible. I think these middle evos definitely fall in line with Game Freak's current design philosophy on starters: drastic changes in third evo and a few hints in the middle evo.