Using tallow on brisket isn't exactly a trade "Texas Secret." People have been doing it for a long time...Texicana BBQ (the guy is from Italy) does this with his briskets. I've tried it before in the past but I feel like it just made the exterior extra greasy, and it also made cleanup a pain in the butt. It obviously didn't "sink into the meat" or anything of that nature, so it was just aesthetics and made the brisket a mess. It did taste a bit better on the tongue, but the chew was the exact same as without it.
You're also forgetting NDA (Non Disclosure Agreements) and how they can have hefty consequences when broken. Also, by all accounts except for John Mueller, Aaron Franklin is a pretty good dude / nice guy. Why would anyone intentionally set out to give away his secrets?