Well, doesn't the rule stipulate that they have to design their own car? Isn't that a sneaky bypass of the rule?
No, the rule is less strict than that. They own the designs to their car from last year, it just so happens those designs were drawn up by a third party in the UK (The Red Bull design team which operates as a separate company). The rule only bans the use of that third party and teams sharing design teams in general.
Incidentally, take a look under the nose of the Toro Rosso. They have a unique way of attaching the front wing.

The Frome Flyer: Smoother, Smarter, Calmer,
Winner.
Jenson Button: Professor, Chauffeur, World Champion Racing Driver.