I'd say it's in house because if it was built by someone else and modified by Mazda, all reviews, specs etc would have mentioned this by now.The unit itself is most likely sourced from a brand that makes Transmissions which was then modified and tuned by Mazda. I’m not saying that’s the case, it may not be, but I’m willing to bet that the units external casing looks identical to the 6 speed that was used in the first generation V6 Mazda 6.
Manufacturers rarely ever build anything from scratch unless they really need to and Mazda is particularly proud of that. They have excellent tech on their cars inside and out which started off with a future proofed design like the L engine. All that’s needed is an occasional evolution of their existing products in order to stay competitive.
The SKYACTIV nomenclature is essentially mazdas older technology thoroughly modified from the ground up, and the result is impressive.