That seems about right... It looks like it travels roughly a meter or so between frames, and the GoPro was likely recording at 60fps, so it was traveling somewhere around 60m/s relative to the parachutist. So add another 10 m/s for rate of descent of the camera and you come up with 252 km/h.

I just spent a while looking up the maths on this and was really surprised to find that the terminal velocity of a spherical rock (diameter 10cm, density 2.5g/cm^3) at this altitude is remarkably close to 300 km/h (I got 340).

Aside: up until now I had imagined that if a skydiver were to ever drop a rock of that size (or a dense piece of equipment like a DSLR) during freefall they would never be able to catch it, but as the terminal velocity of a skydiver in "dart" position is about 320km/h that's not the case. Pretty cool.