Emily Yoffe just updated via Facebook and the LW said the rats were definitely pets, they were in a wicker basket (?!!) and provided more detail on just how filthy the house was (no soap anywhere, dirty clothes all over, shredded paper for cat litter, etc). A commenter said she should decline pet sitting again and call the apartment manager annonymously to complain. I agree!
Edited to add - I took Prudi's response to mean she misunderstood and thought the rats were vermin and endangering the cats. Also I might have misunderstood about there being paper for litter, the way she phrased it is ambiguous.