C919 that appeared to fly dangerously low is more a visual perception due to high power telephoto lens used by majority of the photographers in Zhuhai.
A telephoto lens tend to "pull in" objects in far background closer than is actual case. This is a big contrast than using naked human eyes in judging distance of objects in background.
Thus what appeared to be tree tops near the belly of C919 could be a few hundreds of feets further away to the right side and not directly under the aircraft.
What is the actual height of C919 is hard to determined, unless we have an image that shows the ground directly below the aircraft. Like what T-U-P wrote, C919 could be flying directly above the runway, which is quite safe.