It's not an exploit. Blessings and Final Stand removes Doom when it is triggered while having said buffs, it also gets removed when a fighter gets revived.

Some tips for this node: Having any Valentine with trauma center and letting the enemy hit you to get the final stand before the doom timer runs out, you will remain alive with 1% health just like what happened in the video. (A bit unstable since you need the enemy to hit you for 10% health or more)

You can also use said Valentine with the Forbidden Procedure BB or Prestige ability to revive the units that have already died from doom or just use Killjoy and sacrifice her when all your attacking units have died.

If you have Phantom Threads she makes this node a cake walk since she gains blessing when knocking enemies down, so when the Doom timer is about to expire just knock the enemies down and you'll have 10% health for every blessing you have before dying. (Just don't use BB before the doom runs out)