As Billthedrill pointed out the most likely scenario is a home router. if that was the case (99% likely) then he had no choice but to use the same address as all of the other residents. That's because they all use the address of the router. That is the only address that can be traced. All the residents are assigned an internal address by the router, but that doesn't matter at all and it would not matter if he used an internal address of another resident, it would not change anything externally.
The 1% possibility is that there was no home router, and the provider assigned addresses to all the home users. But in that case he could have used an address from someone across town. It is very unlikely that they would mention the home users without it being the home router scenario.
Maybe more than one? If the other resident had his own router with WIFI, it would be easy to use his IP address by logging onto to his WIFI. And it would be a thing one might try to cover their tracks.