I kept reading this and think, well, not sure how to help. Then finally, this clicked in my mind. That recipe was based on USDA Select. Back in the day when I started cooking them, was the norm and usually all I could get.
I have probably cooked 5 USDA Prime Brisket in 15 years, and I have destroyed all of them, I just cannot seem to cook them. The last 2 were at a Competition last May, and I placed last with both of them, a 1st for me! The only constant with them was they all cooked faster then a Select or Choice.
That is not any help, but all I could think to tell you.
I just did my second brisket using the "basic" recipe with a Costco prime brisket.