Henry had less than half of Tennessee's carries last season (215 of 456). He caught 15 of their 293 receptions.
Yes, you focus on that player much less than you focus on one who has three-fourths of his team's carries (304 of 439) and 77 of 356 receptions.
The Titans had nearly 1,000 yards rushing (1,059 of 2,027) beyond what Henry provided. Dallas had about 500 (1,434 of 1,963).
Defensive coordinators prioritize the players they need to stop. You can't tell me that based on those numbers, Elliott wasn't a higher priority when facing Dallas than Henry was when facing the Titans. You don't play eight in the box against Henry as much when so much of the offense (1,158 from Henry of 5,002 yards compared to 2,001 from Elliott of 5,501) is coming from elsewhere. You study him on film, plan to stop him, etc., but no, he doesn't become the focal point.