LL is car and driver editors and writers running creating lap times. Again they have different driving running different cars on different days in different conditions.Its definitely amateur drivers, with GM sending Jim Mero, same guy that broke the track record (24hour testing) at VIR beating the Ford GT. Not really a fair comparison.
That guy recently retired but spent more time than anyone in corvettes.
I would say the LL times are accurate for GM cars, because they send their driver to do the laps.
That ZR1 time and 720S is close, but at laguna seca its almost a 3-second difference.
Yes manufacturers run lap times too. The zr1 with Jim Mero(who is a pro driver and one of their engineers) ran a lap that was not a lightning lap.