I highly doubt the police could have traced this back to the criminal if he wasn't logged in and didn't use his normal IP, for example, if he had used in private browsing on a VPN it would have taken someone like the NSA to track him down.
Google probably could link the incognito session with the real user session. But would it turn up in a search warrant ?
In this case, they used the IP address, so I guess incognito wouldn't help.