While most flesh magic only affects a single living body at a time, Poppy, thanks to Cain's blessing, can affect multiple people at once. That mass of roiling blood outside was made from the combined bodies of several thousand people that she hunted through the years, and even countless monsters that Cain himself gave her.
Unless Arad is able to understand that, and separate all of the bodies that are entangled in that mess, he won't be able to erase, teleport, or suppress them with magic.
Arad wasn't able to do that, and none of his wives had the power for it. He was stuck; the former Overgod was making sure that it stayed the case as long as he could without getting exposed, because if he came to light, all of the powerhouses hiding in the city would attack.
