This goes on all the time, mostly with cats and sometimes dogs. I've seen far too many houses in the condition described with people living inside. Its hoarding and a mental disease, a form of OCD.
And there is a thing called an "Administrative Warrant" that they could have gotten just to check. It allows the city or responsible agency to enter and check on conditions. It must be backed up with proof but from that article, they had enough to get one.