That's based on this analysis of when teams should intentionally allow a TD when tied.
Aften Thomas gained a 1st down, DAL correctly called a timeout setting up a 1st down and 10 at the DAL 11 with 1:49 to play. This chart illustrates DAL's chances of winning based on field position and time remaining with 1 timeout left.
The strategy of forcing a FG is highlighted in red. It's worth about a 0.01 or 0.02 WP. The strategy of allowing the TD is highlighted in blue. It's worth about a 0.12 WP. Getting the ball back with all four downs and 1:45 to force overtime isn't where you want to find yourself, but it's twelve times better than hoping for a FG miss.
DEN appeared to be completely cognizant that they had DAL in the field goal choke hold, but only after the timeout while Manning and Fox had time to think. I'm not sure Demaryius Thomas would have been so alert in the midst of play. The instinct for the goal line is too great--see Bradshaw comma Ahmad in Super Bowl XLVI. I don't have access to the tape, so it's not certain if that was even a possibility--but that's when DEN should have been prepared to allow the TD.
The right thing for the DAL defense to do is to instruct their players prior to the snap on 1st down and 10 from the DAL 24 with 1:57 to play: If DEN gains a 1st down, miss the tackle.
DEN was able to gain another 1st down, which was the absolute worst possible outcome for DAL. That final 1st down made it a no-brainer, and DEN milked the clock and kicked the FG for the win.