How much effect does the ground have on flight control when flying low?
At the high speed of the F-15s in the video ground effect is almost negligible. Ground effect is about the restriction of downwash by the ground, and at high speed and air density downwash is rather small. Also, the duration of close proximity to the ground is too short for ground effect to fully develop.

The biggest effect will be air turbulence. Since air will neither flow into the ground nor out of it, the vertical speed of atmospheric turbulence is greatly reduced when flying close to the ground. Also, the scale of turbulence will get smaller with proximity to the ground. While gusts may rock your wings when flying high, at very low altitude (less than one wingspan) the turbulence will average out over the wing.

When flying over a ridge, any wind speed component perpendicular to the ridge line will mean updrafts on the windward side of the ridge and downdrafts on the leeward side. This will be the strongest effect in this particular case of F-15s flying over a ridge at Mach 0.7 to 0.9.

Now back to that ground effect meme which seems to find wide agreement here: Ground effect is a stationary process which is strongest at low speed, when the downwash behind the wing is strong. In the case shown here, the aircraft is only momentarily in close proximity to the ground. The pressure field around the aircraft will never reflect that proximity because it is gone before the aerodynamics could start to react. This is completely instationary aerodynamics, and air, having mass, will take some time to respond. However, it is never given this time.