Yhwach claimed last episode Adonaeus was complicit in the sorrow of the Quincy by creating the current world, while here we see compassion for and interference with the sickness of a dying boy, from that same being. Adonaeus is a being of duality (Mimihagi vs Pernida), so both are probably true.

To drink Mimihagi's tears is a metaphor here to take Mimihagi's sorrow into himself, to share its burden relating to the poem from the end of last episode (assuming Uki was the poem pov). "I used to be you and someday you'll turn into me I wish the burden of lives you're shouldering could be eased"