The departure time quoted in the media was most likely pushback time. Airborne time would be 15 to 30 mins later.
On today's flight plan it's about 1:00 flight time from WMKK to the VVTS FIR boundary at DUDIS and another 1:10 to the Sanya FIR at DONDA. So after 2:10 mins flying time they would be in Chinese airspace.
So it must have occurred just before they entered Sanya?
That's assuming they used M771 which they normally do.