You're thinking old technology. Modern units (of which this is one) use multiple radar heads all feeding the same computer system. Furthermore, the incident happened about 5 miles from the airfield which is home to this LARS unit.
Radar itself has not changed much in many years. What has really changed is computer processing of the data. At 5nm it should be pretty good but still not as accurate as a GPS.
The guy's response was very stroppy and rude - totally unnecessary IMHO.
I wouldn't have made that comment myself
Arguing with ATC is totally pointless (except possibly in Spain) and is only going to get their backs up.