My guess would be either:
1) Use AWAC to provide intercept coordinates. But this was during the late 90s/early 2000s when China didn't have many AWACS, so maybe
2) Flanker's wingman opened its radar to scan for the J-7 and relayed the data to its partner or
3) The pilot used IRST to pick...
Not a fair comparison since the Viper is single engine whereas the Flanker is not only twin engined, but the engines are spaced very far apart, which gives it a much larger angular momentum to overcome during rolling maneuvers.