Did you read the article? The house houses two families. It is on a corner, I.e. facing two different streets. Each of the two entrances, to the two residences, had a different street address. It was the other tenants at the other address who hung the Nazi flag.
Perhaps they share the same entrance, but if not, it may also be the case they made a stupid mistake, thinking they were being sarcastic with the flag, and it backfired on them, so now they are claiming innocence.