I would push back on that.
Old school Texas BBQ joints like Louie Mueller, Black's, and Cooper's have been cooking amazing briskets and clods for 50+ years without tallow, most of which are/were choice. For the longest time when I got into brisket smoking in 2008 or so, people actually sought out better grades of choice over prime brisket because prime had too much fat and less beefy flavor. Though I do personally think prime is considerably easier to cook than choice.
Rich is an arbitrary/subjective word when it comes to flavor. Hell even Meat Church did a video a while back where the guy that got the tallow craze going (Jirby from Goldee's) where he says using tallow makes the brisket more "rich" but also a little "bland." Maybe I am on my own island here, but I think of rich flavor being intense and bland being the exact opposite. Not sure how it can be rich and bland at the same time. I think it's just that fatty coating that lines the pallet and cuts my ability to taste flavor that people are calling "rich."
Beef tallow is very bland to me. Again, if you want to see what flavor it adds to the meat, just warm some up and taste a teaspoon.. tastes like crisco to me. Would be great to fry up some tater slices or something I bet, but I am not putting crisco or tallow on my brisket again. I think it makes it really shiney for pictures, but I'd rather just use butter or bacon grease if I want to add an enjoyable flavor.
Render the fat, create a good bark, and cook until tender. I have had and have cooked dozens of choices briskets from Walmart, Kroger, Albertsons and everywhere else over the last 15+ years. No thanks on the tallow. I did it once and didn't like it at all. If I am drying one out during the cook, which happens, I just wrapped in foil vs paper to hold the moisture in and sacrifice bark crunch a little.
My take is it will certainly add moisture to a dry brisket, but so will injection and wrapping in foil vs paper. If strong beef, smoke, and rub flavor is not your thing, tallow will cut it back and you'll enjoy it more than a traditional.
See 9:45 mark in video below for "the tallow is a little bit more rich, but a little bit more bland" comment from Goldee's owner.
No Tallow vs Tallow Meat Church