I don't always agree with the Death Battle rationales, but this one makes sense within the canon.
Except:
spoiler space for the video
The DB team themselves had pointed out that Banner and the Hulk help each other survive, and strictly speaking the Hulk has always been the most dangerous and hardest to defeat when his personalities are unified. That's true for the so-called Worldbreaker Hulk, too; in the Sentry vs Hulk battle they referenced as an example of their victory theory, the Unified Hulk essentially allowed himself to be defeated to keep from breaking the world. And also hadn't entirely fought as a unified personality. Had Banner's super-intelligence and the Grey Hulk's tactical craftiness been combined, the fight might have gone differently.
(Maybe. They're right about Doomsday being able to ramp up to Superman levels of speed, at least for short tactical bursts.)
On the other hand, Banner might have theoretically accepted things had gone too far into a risk of a worldbreaker situation, and so surrendered despite being killed, on the hope that sparing the planet now would give someone else time to stop DD.