Triple-A is a designation usually given to a game, rather than a publisher, as it reflects the amount of resources and polish a specific project received.
Triple-A is a designation given to games which receive some undefined large amount of funding toward its development. That tends to correlate with being developed or published by a large studio, which Dave the Diver definitely was, but MINTROCKET wasn't given nearly the same resources as, say, a typical Xbox Games Studio studio would be.
Not even that. What matters is the budget and the team size, not the size of the studio. Rockstar Table-Tennis was not a AAA game despite coming from the AAA developer.