But at 160° it should be brown...

Very thin burgers..

Just by the photo(and hard to judge by that) looks thicker than Bar-B-Lew's. Not sure of how his was ground but you can see the individual hamburger grinder plate strands. Maybe he didn't compress his as much. His you can see the more usual browning on each edges or corner of each half. I agree in yours, that is not showing. With what king of prob did you test the temp with? Could you have inserted it to far and tested the other outside temp instead of center or it was just to thin to get a center temp separate from the outside temp? Not sure how a temp probe works but I don't think it only reads the end 16th of an inch I would assume it reads temp for at least 1/4 inch so hard to get reading on thin meet. Did you mix anything into the Hamburger before cooking? I also have a Danial Boone. When you say indirect you must have the 2 piece drip tray? I only have the one piece. At that temp I would not think it was smoke ring all the way to center. I would just say it was very rare but up to temp were the temp probe checked. But as long as it was good.

Mudflap

 Sometime back I used to smoke hamburgers..I don't exactly recall the cooking method other than they were smoked for a few hrs, then cooked briefly at a slightly higher temp. The done temp was 165º.

 The meat was smoke ring red all the way thru. Good taste and moist. 

The meat was smoke ring red all the way thru

Just curious
I always thought to get a good smoke ring you had to cook at a very low heat for a period of time not at 400 degrees.  If I am right how did the OP obtain that smoke ring as he  stated he cooked his burgers at 400 degrees.

Cooking temp is not what matters, although has an impact. Smoke ring will form as long as the meat is below ~140 - 160F.  Pellet grill don't put out much smoke at 400F, but they probably produce plenty of NO at that temp.

After reading all of the responses again I am not sure we will ever know the answer as the OP ate the evidence.

Did the OP say what wood was used? Cherry can cause very red meat color.

Did the OP say what wood was used? Cherry can cause very red meat color.

I've seen cherry change the color of the outside of the meat to a reddish tone, but never the interior.

Looks like it's been corned.  ?

Wood was hickory and Oak.

Pink slime. Maybe the beef came from the same provider that sells meat to school cafeterias.

Another possibility, maybe the meat was travelling toward your mouth at such a high speed that there was an observable red shift in the visible light spectrum given off by the tasty burger morsel.