That’s in the comments of the Part 1 post, linked before the post body, if you missed it.

She was found with a brood patch, which develops to incubate eggs and keep the very young owlets warm. They have a patch of skin that loses feathers, gets increased vascular activity, and the skin slackens so it can get maximum contact on the eggs/hatchlings.

So momma owl’s original offspring have likely failed at this point, as it was impossible to find her nest with her trapped and injured. Getting united with this baby fills in the lost roles for both parties, so for the most part, they will both get to continue where their lives were interrupted. Baby needs a proper owl momma, and momma was primed to raise owlets.