Yep and due to the same reactions that cause cured meat to turn pink when cooked.

Also agree that the brisket seems to have been pulled way earlier than your typical smoked brisket, but to each their own. I base most of my cooking on the science behind the common techniques, but at the end of the day if someone wants a “rarer” brisket and they ensure that it is cooked properly to avoid food any illness then go right ahead. For example, I believe (through the science and my personal tastes) that a good cut of steak is often best enjoyed medium rare with a good clean sear. I don’t like preparing those meats rare and certainly not Pittsburgh style, but others clearly do.

I like to challenge my friends when they have particular doneness preferences to at least try my way (especially when it’s something like wanting beef tenderloin medium or well done). I cooked up a tenderloin roast for a group that all wanted theirs well done, but I insisted they try it just at medium rare with the promise of cooking theirs further if they didn’t like it. Nobody asked me to do that..